To answer the question if OnLive only let users play multi-player games with other OnLive users then? Yes, multiplayer is only with other OnLive users which is more secure and makes sure that all have great multiplayer latency, as OnLive multiplayer is basically a giant LAN because you play with people who are on the same data center or connected through the internet backbone. So, all can have a great multiplayer experience.
But, OnLive have stated that they can support multiplayer with PC gamers or other platforms. It's up to the game publishers to decide upon this. To date no publisher has opted this way because of the many benefits that OnLive's kind of multiplayer brings.
The publisher of EVE Online has stated that they are talking with OnLive to bring EVE Online to them, so it might happen that OnLive will get an MMO game with support for multiplayer gaming with PC and console gamers.