According to the bug filed about the problem, "It would appear that Android is, at some level, interpreting specific text strings and acting as if they were local commands," according to user called mogphone.
Added another commenter, jdhorvat, "Funny story behind finding this: I was in the middle of a text conversation with my girl when she asked why I hadn't responded. I had just rebooted my phone and the first thing I typed was a response to her text which simply stated 'Reboot'--which, to my surprise, rebooted my phone."
Repairs are under way. "This is already fixed and is going out in the RC30 build which will be pushed to users very soon," added user morrildl.
Indeed, my Android phone got the RC30 patch over the weekend, and the problem doesn't occur. It's not clear what time frame other Android users will receive their patches; there was a multi-day lag between my G1's last update a week ago and the period in which some others got their updates.
Some more details on the problem are described at this Web site.