Various online Spanish websites have posted scans from the French Playstation 2 magazine of what looks like a very impressive looking Splinter Cell Essentials for the PSP.
Information gleaned from these websites tell us that
Essentials will feature nine missions of Sam Fisher's whole carreer as a Splinter Cell (two remakes and seven brand new), one missions which predates Fisher's work as a Splinter Cell Agent, and better graphics than a PS2 version of Splinter Cell. There are a few more features mentioned such as a possible WiFi mode and 60-player team play, but I'd take those lightly.
If you don't have a PSP now, this game may be the gentle nudge to go pick one up. The estimated release date is tentatively Q1 2006.