My new Mac Mini 2010 worked fine when it arrived a couple weeks ago. After migrating my 10.5-based apps and data, I got the not-awaking-from-sleep problem. In my case, it's almost like it shuts down automatically, since tapping the power button immediately causes a system start. I reinstalled the original 10.6.4(?) system, it worked fine. This time, I decided to migrate software incrementally. Started with my Mail folder. On opening Mail.app, it insisted on IMAP access to my gmail account. I canceled this, restored the system, and copied my older keychain.login (after validating or checking or whatever from within the Keychain app), with the POP gmail access. Mail worked fine. However, the sleep problem returned. I repeated the process (restored system, worked OK; updated keychain, this time without even launching Mail. Sleep problem was back. Restored original system again, worked fine. Copied the Mail folder and just the 2 older keychain items needed for my Pop and SMTP mail login; again, I didn't even run the Mail app. The sleep problem returned.
And that's what I plan to do with the Mac--return it! I've been a Mac user, nay, enthusiast, since the SE, and have NEVER had these kinds of issues. I'll get an older system that'll work fine with 10.5, and am actually looking into Windows 7, which works (and looks) pretty nice on my computer at work.