Is this possible with an early 2009 iMac without Thunderbolt and a 2011 MacBook Pro?
My dream solution looks like a Apple Thunderbolt Display!
No, as the first iMac which allows this is the 27-inch Late 2009 iMac, as explained here.
There are, though, some Applications which may achieve something similar, requiring both computers to be connected to the same LAN/WiFi/VPN, and sending all the information, the one to be displayed on the second computer working as a display, through it. Such as ScreenRecycler. You can find some more here.
Yeah, you actually can. Check out a program called Screen Recycler. It costs money, but the trial will work for long enough.
Also, check out air server and air parrot. If you run air parrot on the macbook, and connect to the iMac with air server, it will also work.