I have a PowerEdge 2900 which I suppose uses the same backplane as the 2950. I ordered it with 2 SATA drives (250GB). The are in a a caddy that is marked with "SATA + SAS" and they have the "interposer" board.
Then I ordered a 80 GB SATA drive, since it seems the cheapest way to get the caddy here (EUR 45,- for drive + caddy). It came only with the harddisk (no interposer), and was marked with "SATA" only. The screw holes were obviously at different positions, so that the bare drive still connects to the backplane.
Both solutions work, so I see no difference.
Just my 2c, __ /homas
BTW: where can one get that caddies - cheaper than EUR 45,- (around $60) - in Europe?