I don't know of any standard GI component functionality to do that, but honestly have not dug into it to see.
A possible choice would be to integrate a jquery component that I was assisting someone on the other day that has the exact functionality you are looking for.
Here is a link to that component and I do not know what implications it might have in using it with GI, I know it will take some javascript manipulation to make it work in the GI environment.
http://dropdown-check-list.googlecode.c ... klist.htmlHope that helps,
Ed