I think that'd be a great feature for one of the big boys (i.e.: Google, MSN, Yahoo, PageFlakes, etc.) to support the others' widgets as well.
But, in the real world there is a problem with that though. Just like when an operating system supports applications from another, there comes a time when the developers wonder why they should create anything for OS A when all of OS B's applications will work on A.. (Just refer to OS/2 for more details..)
I'll add some examples and basically a bunch of test-cases for it soon.