I realize this is an older thread, but I am posting because I believe I found a "solution" to this problem. At the very least I found a useable workaround.
So, my sister had a Droid 2 and it did this to her repeatedly. The final straw was when she attended a wedding and the ceremony was interrupted because she got a phone call (from me ironically LOL), and of course the phone had powered itself back on after she shut it down. She was so upset she gave me the Droid 2 and paid almost $1k for an iPhone (she was a year from being up for her upgrade, so had to pay full retail).
When my sister owned the phone it was fully stock. She's not very tech savvy. The first thing I did when I got the phone was SBF it back to factory original, root the phone, and then install Cyanogen mod 7. I'm now running CM 7.2 RC1 and guess what... when I power the phone off, it STAYS powered off.
So, this may not be the news everyone wanted to hear, but rooting and installing a custom rom on your phone will fix this problem. It also fixed several other issues she experienced, such as unresponsive and overresponsive (as in extra letters, or getting letters without even touching the screen) input while using the soft keyboard. The phone used to compose texts all by itself, launch apps on its own, have apps force close with errors repeatedly, etc. while she owned it. With CM on it? It never happens.
I know of several people who were extremely dissatisfied with their D2... but all of them were running the stock verizon roms and just letting upgrades happen as they were pushed down. Rooting and customizing this phone has made it so much better to use. As a bonus side effect, no motoblur means it runs WAY faster than it used to, as well. I'm now due for an upgrade myself and I want a 4G phone, but I'm holding off because none of the current Verizon phones are tempting enough for me to discard the D2.