If you use your MacBook Pro as the "server" connected to the Internet. you can connect 27" 2011 iMac to your MacBook Pro via a FireWire 800 cable. (assuming your MBP has a FireWire output, I don't own one).By using Target Disk Mode (Hold down the "T" key until you see "spinning gears" on Restart or Start-up).
Your MBP will appear as a Hard Disk on your 27" display or your 27" iMac can be displayed as a Hard disc on your MBP display.
- Switch the firewall off on both machines (if it was on) to see if it helps;
- Try to connect using a different device to the shared connection (another computer, smartphone, iPad, etc.);
- Use an open connection first (no password), before trying with WEP;
- Reverse the roles of the two computers (i.e. try sharing the connection from the MBP to the iMac).
Also, more details might help (OS X version on the machines, whether you are trying to share a wired connection, installed software which might interact with the network connections, etc.)
Hope this helps in some way.