I have a Motorola XT720 on eclair. A couple times, I've seen it show the boot animation (mystical Motorola logo), usually it's while it's sleeping. After I see the boot animation, I am on Emergency calls only, just like when I first boot my phone. (I'm not sure if this is normal either.)
I just saw this now, and it was plugged into my computer (usb debugging enabled). I had it set to memory card access and ejected it and switched to portal & tools. It sat for a couple minutes and then I saw the logo. I touched the screen and it showed my lock screen.
I'd assume this is a crash and reboot, except that if I goto Settings > About phone > Status, I see the uptime listed as 116 hours.
What is the phone doing? Does anyone else see this behaviour?
Update: I read elsewhere that it may be related to the SD card and I've started fscking whenever it happens.
First I find out where my device is connected (I'm on /dev/sdb1) and then I fsck.
grep fat /etc/mtab sudo fsck.vfat -r /dev/sdb1
fsck will do interactive repair and ask you how to fix the errors. After I get one of these crashes, there are errors to repair. I think this has reduced the crash frequency.
Update 2: I put in an old 1 GB SD card awhile ago and I don't remember the last time I saw one of these crashes.