Question: I keep accidentally dragging windows into other workspaces on my Macbook Air running OS X Lion.

Can I disable dragging into other workspaces or make doing so more difficult?

Answer: 
TinkerTool has this option in the Applications tab, Mission Control, Delay when dragging window to adjacent space: Default / Short / No Delay.  

No Delay was selected on my computer, but I don't remember if I chose that at some point. I looked into this because I wanted the "windows aero snap" feature, I just found and installed [Cinch by Irradiated Software](http://www.irradiatedsoftware.com/cinch/). The Spaces "swipe" would happen before Cinch was able to do its thing.
With the [Warp pref-pane](http://www.ksuther.com/warp/), you can set a delay and/or require a modifier key(s) to allow a space change.