Yep.
And you also need a Thunderbolt to Thunderbolt cable - which doesn't exist yet.
So to answer your question OP - No. The 2010 MBA Will never be able to use the 2011 iMac as an external display.
Actually, yes, and no.
No, your 2010 MBA will never be able to use the 2011 iMac as an external display,
via Thunderbolt.
And yes, your 2010 MBA
can use your iMac as an external display,
via Air Display, a wireless external monitor connection. In fact any Mac with Snow Leopard is capable of this. This is an awesome app, as I use it the other way around as you do; that is, my MBA is used as an extended secondary monitor for my iMac. Your second Mac will act exactly like a secondary display under system preferences. You can mirror, extend, and adjust arrangement however you'd like.
I've been using it for a couple of months now and its beautiful. Works exactly as advertised. It's still a bit buggy but it keeps improving with software updates, and I can tell you that it definitely works way better than it did when they first launched Air Display for Mac. Also, you'd think video content wouldn't be watchable over a wifi connection, but you'd be surprised
Highly recommended.