Boy you must have a ton of stuff on your phone if your internal storage is that low. I realize I just started using my phone a week or so ago, but right now my internal storage is 1.03gb and my system cache is 310mb. I do however have a 32gb sdcard installed and I use it too store everything so maybe that's why I haven't used that much. I also stripped most of the junk apps, sounds/videos/bootanimations, etc. out of the system folder after I rooted so that may be a reason as well.
I'll have to keep a close eye on my storage space as I start adding things to my phone just to make sure I don't start losing space like you have. Maybe there is a problem with these phones not recovering used space properly.