Actually, one of the channels does this - Gabby. They call it "screencasting." It's not fantastic, but it does work. Takes about 30 s to a minute to buffer (needs that time to save enough screen captures and transcoding to stream). Works over g wifi. When I do it, resolution/clarity is not great, but it does provide a means of streaming Flash sites.
The other nice thing about Gabby is that it allows you to stream most (common?) video formats, since it transcodes on the fly. No more need to convert files.
I just tried Gabby. I can see how it might serve up videos from a Windows PC on your network, but I don't see the screen casting option. But, alas, it simply didn't work at all. Got it connected, my Roku saw my server, but when I click on it, it just says "retrieving..." forever....