I bought a really cheap computer on sale at one of the on-line shops - paid $9.99 for it, I think. Then I set up a mount for it on my trainer with Joe33's help. I think it could be set up with spit and bandaids, if it had to be. It is only for indoor miles and my main, front wheel drive, computer sits idly by.
I got the idea from Joe33 at bikejournal, he posted instructions for his fancy system on his blog.
Here's his blog address, but I can't find the specific post

http://joesieracki.blogspot.com/

Anyway, you can't see it while you are riding, but I ride indoors by HR and cadence. After you are done, you can hop of and see how many "miles" you have ridden. The cheap computer I bought gives elapsed time and miles - doesn't even give average miles/hr, but that can be divided out easily enough.