I dunno about a sleeve per say, but I have a Kenneth Cole notebook messenger type bag and it works great for the sentia. It's actually made for notebooks with up to a 14 inch screen, so it has extra room. The inside is very nice, and it has lots of pockets and nooks for all the extra stuff you are bound to carry.
Anybody come up with a sleeve yet? I am looking too.
I am going to ask my local upholstry shop to see what they can do. They have made some really nice insert bags for my Harley Davidson out of wet suit material. That may be just what is needed for the sentia.