Earlier this year the PSP enjoyed a rise in sales due to the release of the slimmed down PSP-2000, as well as an assortment of AAA titles, such as FFVII: Crisis Core and God of War: Chains of Olympus.
Since then, there hasn't been much fuss over the PSP, and why would there be? Where are the games? Not much worth picking up has been released since Q1 08'.
Sure things are bound to pick up again with the release of Resistance: Retribution, Super StarDust Portable, etc.
Sony needs to find a way to keep the sales, and interest going steady for the PSP. With the PSP being a technical marvel, it shouldn't be too difficult to implement some console-like features to it, to make it really shine.
The PSP needs full PSN integration. The PSP needs a full PSN friends list, where you can message, initiate a skype chat session, etc. We have a PSN "Portable ID" yet we cannot use it on our "portable" PlayStation.