Joel, there are lots of places to go fishing out West that are all awesome. For me it comes down to time and money. I want to get somewhere quickly and not spend an arm and a leg. For me, Colorado fits that well. Frontier Airlines now has great (and cheap) service out of Knoxville to Denver. I can fly out at 9:30a ET and be fishing in the Flattops by 3p MT, for less than $230.
As far as picking particular spots, you need to do research, research, research (which to me is almost half the fun). One of the best investments you can make are the Trails Illustrated maps (the same ones we talk about for here in the Smokies). They are an excellent way of getting to know where the blue-lines are in a particular area and what is and what isn't public land. I have way too many on them
. There are 4 of them for the Flattops (NE, SE, NW and SW). If you come in from the Dotsero corner of the Flattops Wilderness (I-70 before Glenwood Canyon), and travel up Coffee Pot Rd, you can access Deep Creek, Grizzly Creek, Heart lake, White Owl Lake, Deep Lake, Buck Creek, Doe Creek and the upper end of the South Fork of the White River (around Budges Resort). In other words, lots of water all in close proximity (you could even fish Gore Creek and Eagle River on your way down I-70 from Vail).
Bob, I completely agree about the NW side of the Flattops (and the whole west side of the White River NF, for that matter), public and private land mix together all over the place and its not well marked. Like many of the larger rivers, the White River doesn't have much public access water once it gets to be a major river. That's what's so great about the Gunnison area, there is a lot of great access to big water. There are a few areas in and around Creede and South Fork, where you can fish the Rio Grande, as well.
However, I tend to stick to blue-lining as much as possible, and the best way to insure that you are on public lands is to fish inside of a Wilderness Area. The three that I have spent the most time in (in CO), are the Flattops, the Wemuniche and South San Juan Wilderness Areas. The Wemuniche and the South San Juan are better fishing (IMO) than the Flattops, but are much harder to get to from the Denver Airport (they are at least a 5-7 hrs drive depending on where you are going, whereas the FT Wilderness is less than 4hrs from DIA). The only other problem with Wemuniche and South San Juan is that they tend to get a lot more rain than the Flattops (although that was not the case this past week). I spent 36 hrs in a 2 person backpacking tent in 2008. Monsoon season can be really tough. You will get wet in Southern Colorado. If you plan on it and prepare for it accordingly, there is no reason why you can't keep fishing.
In addition to the TI maps, I also use the Forest Service Maps for the national forests that cover those wilderness area, as well as the Delorme Atlas and Gazetteer for Colorado. Mine is a very dog eared copy that I've had since 1991. I've made all sorts of notes in it over the years and have highlighted all of the streams that seem to be worth fishing (and have been told by others are worth fishing-there is still a stream that DK turned me on to that I will get to one day, as well as a few that DD has shared indirectly and directly with me).
"Fly Fishing Southern Colorado", by Craig Martin and "Hiking Colorado's Wemuniche & South San Juan Wilderness Areas" by Donna Ikenberry are two great books to help you find (and quantify the effort needed to hike to), some real gems.
I also use technology, as well. Its amazing what simple internet searches will turn up ("creede colorado fishing", for instance). Then there are all sorts of guides who post fishing reports covering the streams across the state, so you can get an idea of how the major rivers are fishing (after all, blue-lines are all tributaries of larger rivers). Most of the fishing reports also talk about the small streams in their area, at least in general terms. If you've picked a wilderness area, sometimes the best thing to do is go to one of the flyshops near the area , when you make it out there, buy some stuff and pick the owners/guides brains as to where to go...they are almost always willing to help, especially if you patronize their store.
Another invaluable tool is Google Earth. Once you find a location on a paper map that looks good, you can drill right down to a detailed terrain view and map out where you could setup camp and how far of a hike you should make. You can even get a sense for how rugged the terrain in the area is by moving from a horizontal map position to a birds eye view.
But let's simplify this a bit. The reality is, that for the most part, most blue-lines in wilderness areas will contain an abundance of frisky fish. If you are above 9000', you can usually find Cutthroats and Brookies. Sometimes even Rainbows and Browns can be found at that elevation and higher. Like streams here, the higher you go, the smaller the fish will get. But in most cases the blue-line fish will be between 10-12", with some smaller and some larger. Sometimes you will find a stream that holds lots of 16" fish, but that will be one that is a long hike to get to...to be sure. And size is not always a sign of potential performance (
) I've fished two small streams out there that averaged less than 3' wide and every pool had lots of 10-14" fish.
The bigger streams may hold bigger fish, but not always. The bigger streams can also get very technical, whereas most blue-lines are yellow stimi or xcaddis heaven. Don't be afraid to high stick up a stream that looks too small to hold fish of any size. I bet you'll be amazed at the size of fish that can survive out there...
That was probably way more info than you wanted...