I suspect that if you fly Delta or Continental, you get a jetway, but if you book thru them but actually fly on a partner(Air France) then you will get the bus.

We did use the CDGVAL from the RER-B stop for T-1 to reach T-1. It was great! I was a little confused in T-1, however, due to the terminology for the halls, gates etc. (of course, now I can't remember what the terminology was).