Say I have 1000 static items in an esp with the ID prefix "aa_" and I want to change them all to prefix 'bb_'
or even just add a new prefix to everything in an esp.

Does anyone know of a quick way to change multiple id's like this in one go ?

I think http://code.google.com/p/mlox/wiki/Tes3cmd could do something like this if you are able to use regular expressions (or kindly ask John for help)
[EDIT]Or you may be lucky if your aa_ prefix is unique to your object ids inside the mod (you can verify this with TESCS or MWEdit search), you could just search/replace your file with any hex editor (I suggest PSPad as general purpose text editor).

I generally use keyboard macros to do repetitive tasks like that. AutoHotkey is free and has macro recording capabilities included.