The connectors should be the same on the S1 and S2, but only the S2 is MHL compatible.
I think MHL can use any 5 pin connector to send an HDMI signal while receiving power for charging.
I'm not sure if you'll be able to use the MHL HDMI out and USB at the same time, but I hope this is possible.
I'm also guessing that the adaptor will need to be a clever one with circuitry rather than a passive adaptor. Does anyone know if this will definitely be the case, and if so I'd imagine it'll have cost implications! Hope I'm wrong and a passive adaptor is possible
Current phone: Nexus 5
Previous phones: HTC Kaiser, SE Xperia X1, HTC HD, HTC Pro2, HTC HD2, HTC Desire Z, Galaxy S2, HTC One X, Galaxy Note 2