After reading what Christian wrote about the solutions manual that he and his collaborator wrote for their Bayesian Core book, I’m reminded of my own efforts with the Bayesian Data Analysis solutions. Not long after the first edition of the book came out, nearly fifteen years ago, I wrote up solutions to fifty of the homework problems (out of a total of about 150). People sometimes ask for more solutions, but the difficulty is that, once you have official solutions, you want them to be correct, you want them to be clear, and you want them to illustrate good statistical practice. It’s a lot of work. Somehow I was able to write up those fifty, back when I had more time on my hands, but, really, writing up another fifty would almost be the equivalent of writing a (short) book! Originally I thought I could quickly put together a complete or nearly complete set by gathering solutions from students, or people just emailing them in, but I quickly realized that this wouldn’t work. I think it would be ok to post scanned-in versions of student solutions, but once I start typing them up, I need them to be cleaner, and that takes work. That’s one reason I didn’t even try to write a solution set for ARM.
I typed up solutions for every question in Part I (first four chapters) a couple of years ago. I can't vouch for the correctness of the ones not already in the solution guide (and I know I have one wrong in the ones that are in the solution guide, but I couldn't spot the error in my algebra), but you're welcome to what I've got.
another possibility: start by writing the complete solution manual, then write the questions, and only then write the textbook.
One solution would be to open up a wiki for the purposes of letting others post their solutions, and you could review the solutions and "bless" some of them as correct & good. If you're too much of a perfectionist though, you'll be too tempted to re-write them all. =)
Encourage your best students to write up their answers in LaTeX. This is roughly how the Decision Theory book by Parmigiani, Inoue, and Lopes got its start (in that case, IIRC, it was Lurdes and Hedibert's class notes).
The idea of a wiki with more than one solution per exercise is really great (but assuming that not all solutions will be fancy and thus not pedagogical).
Something's better than nothing. How about "detailed hints" to groups of problems in several blog posts? If it's on your blog, it's not official, and there's little motivation to make it perfect. Those who care can add comments. Having solutions to the problems, or even just a nudge in the right direction, really does help.
There may be some sociotechnical obstacles to working in a wiki with a lot of mathematics in it, but you could perhaps make a lot of progress by setting up a public repository of the TeX files at github (or bitbucket, or one of the other social version control services).
However, if solutions are written in nice and clear way they can be even more valuable than the book, beacuse books often skip important parts of "the trade" that are essential for solving the real life problems.