iphoneexcitor

my iphone is 8GB and on 1.1.3.
as you all probably know, it says 8Gb but the actual usable size is 7.2GB. then after upgrade to 1.1.3 it's down to 6.9GB. today when i checked, for some reason, it's down to 6.8GB. and i didn't add any new songs or anything.

i install and uninstall apps alot. my question is, when i click the uninstall button, does it actually free up the memory? i have term-vt100 installed so i guess i could go into the system and manually delete some files if they are not deleted. i just don't know where i should be deleting them.