Just a rumor for now, according to World of Apple and My Apple Guide (via Engadget), and it will likely be a "Spotlight Turns to Desktops" with architectural bumps for the Mac Mini, iMac, and/or Mac Pro to bring them in line with the Nvidia chipsets and latest gen Intel procs the laptops received last fall, and maybe a new 30" LED display, but March 2008 did see the iPhone SDK Roadmap event, so we're not above hoping there may be some iPhone news on our horizon.
When Apple announced they'd no longer be attending Macworld, they also said they would be calling their own news events on their own schedule, so likewise the March 24 data has to be seen as fluid and depending on the final readiness of the product(s) and service(s) to be announced. Since Apple rarely sends out invitations until less then a week before the drop date anyway, all we can do is wait... and maybe drool...
Won't be Steve Jobs, of course, as he'll still be on leave, but Phil Schiller did the last keynote, and also the last new iMac form-factor reveal (while Jobs was on his previous leave) so there's some history to suggest he may be the main man on stage again.
Meanwhile, what do you expect to see at a March Apple Special Event, and what do you really want to see?