OK so I may be able to shed some light on this as I had to support windows mobile since the 6601....
if you have an app running that uses a data connection (IM, pop3/imap email,weather app) and you are suspended (ie
ower button pressed to turn off) and you loose that connection.... some apps will imediatly try to restore that connection when it can. if this happens the ppc will "wake up" long enought to connect to the data network. this is fine id you have your phone on the table but sucks if it is in your pocket.
Another way is if you use ActiveSync to an Exchange server. same as above except you can also have it wake up when something changes in your mailbox (you read a message/add a contact). Exchange sends a "wake up and sync" message to the phone and it complies.
without a physical lock switch like on Treo WM device I am not sure how to avoid this issue.
Now, if someone could figure out how to re-assign the WiFi switch to be a device lock that would kick buttocks!