I noticed an interesting behavior in the iPhone's Gmail implementation. When I delete a message from the inbox, I expected this to behave the same as it does with IMAP, where deleting a message from the inbox just Archives the message.
But in the case of the iPhone, deleting a message actually deletes the message, (moves it to Trash on gmail). So if you want to just archive a message you need to move it to the "All Mail" folder.
It looks like this has been documented.