Palm has current relationships with the three biggest American cell carriers, plus channels in Canada, France, Germany, UK, and Mexico. Palm also has an extensive patent portfolio, many of which Apple is infringing upon.
However, the real prize is WebOS. I just got back from Palm's Developer Day, and their technology stack for their next OS version is pretty damn neat. It's a high performance event-driven javascript architecture on the back end, with WebKit on the front, all programmed through a Rails-ish MVC model. As a web developer, it looks shockingly easy to program for.
Of course, if you want to program the phone in raw C/C++ and SDL, you can do that too. It's incredibly flexible.
What Palm has been lacking is cash and time. The WebOS development stack won't be fully built out until Fall, and they haven't had the resources to push new phone models out as quickly as they need. This deal gives Palm much needed juice, and gives HP all the things they need to go head-to-head with Apple.
If HP is smart, they'll start tossing WebOS on tablets as soon as they can. I'd buy an HP slate with WebOS in a heartbeat.