I have a moto droid
Its possible but it is not ideal, IMHO. My old Moto Q WinMo was better at this. I'd like to be able to bookmark a favorite and put it into an existing or new folder on the fly. Having a ton of uncategorized bookmarks looks bad whether in list or thumbnails view. I'm actually suprised more people have not complained about it. So much so, that I think there might be a way and I'm just missing it.
Here's what I do, for the time being. I create new folders on one of my home screens. This home screen is just bookmark folders. For example, I have a folder called work. I bookmark sites the normal way. I then go to this home screen and long press to add a shortcut to this bookmark. It appears on the home screen. I then drag and drop it into the folder called work. So instead of ever going to the actual "boomarks", I go to the folders on my home screen that have all my boomarks categorized. Its kind of a pain in the arse. I think someone will fix it, if they have'nt already.
=Scott