I too would like to know more about this issue - it is my understanding we can only save to the SD Card and Internal, and have zero control over phone memory.
I came to the boards today looking for a reason as to why I cannot update a large app (27MB) when I have over 600 MB available on the phone memory, 1.2 GB on my SD card, and 6.5GB available on internal phone storage.
This phone is starting to piss me off, and Verizon is making me more so. Rooting becomes more and more attractive every day.
While I didn't answer your question, I hope you don't mind my leaching onto it as our issues are similar in content.
/rage