Keith Reeder said:decide whether you're happy taking the tripod approach (bearing in mind that you might end up having to lug two tripods around, one for camera, one for scope. It'd be a royal pain in the buttocks to have one tripod doing "double duty"), or whether you want a camera/lens combo you can use handheld.
If it's the latter (and as you know, it is for me), then a big prime is really not the lens for you - the "bean bag/leaning against the wall" approach would not do it justice, and you'd be much, much better off with one of the slightly shorter zooms mentioned already.
While I agree that it's important to decide how you're going to use the lens before you decide which ones right for you... I don't find that using the same tripod for the scope and the camera is a problem. It might mean that I miss some shots because I'm not set up (as the scopes on the tripod), but I'm a birder first so woudl rather have teh scope ready than the camera.
As Sean has mentioned he uses the Sigma 500 prime handheld and gets some excellent results (makes sure you have a look at his gallery). I've tried the lens handheld, on a beanbag, with a monopod and with a tripod. My best results are with a tripod, but with a monopod or beanbag I get much better results than my 170-500 zoom gave.
That said when you look at some of the shots taken with the 170-500 and the 135-400 lenses (take a look at Keith's gallery, esp the recent goldcrest shots), you realise that less-expensive lenses can still deliver great results in the right hands.