Various people have labelled "Charlie Murphy" as the voice of Jizzy, the 'adult entertainment' club owner in San Fierro. You can hear Jizzy speak via the downloadable audio file on the official website (Visit San Fierro and select "Jizzy's Pleasure Domes" in Juniper Hollow).
For those who don't know, Charlie Murphy (Eddie Murphy's brother) is a comedian on the Dave Chappelle Show.
Gamespot reckon that Sweet Johnson sounds an awful lot like Ice-Cube, whom we heard in the official trailer yesterday. (Ice-Cube was spotted outside the Rockstar Offices several months ago)
We've yet to put a name on Ryder. If you have any suggestions, by all means, contact me. And please remember, none of this information is confirmed.