What has Arsenio Hall been doing for the last decade or so?

The short answer is: Sleeping on a big pile of money.
Arsenio (very quietly) had one of the most lucrative TV contracts ever, and has maintained all of his wealth through shrewd investments. He’s been quoted as saying the intense spotlight of celebrity was overwhelming, and besides a few cameo appearances here and there, he spends most of his time behind the camera producing various TV/film projects.

Looks like he has been doing a lot of voice work for cartoons too:

Igor (2008) (post-production) (voice) .... Carl Cristall
Scooby-Doo! Pirates Ahoy! (2006) (V) (voice) .... Captain Crothers
The Proud Family Movie (2005) (TV) (voice) .... Dr. Carver/Bobby Proud

Good for him. I liked Arsenio, a very smart, shrewd, cautious, albeit controversial individual. Arguably with good cause. He knew what he wanted, went in there, got it and then left. Way to go . Sometimes it is not prudent to hog the spotlight because it can work againt you. I wish him well. Enjoy your success peacefully. Bravo Arsenio.