I'm pretty sure that Pearson, the leading provider of K-12 software built their new Power-Teacher product, using Flex:
Warning, that's a 15 minute, 58 Meg video. But it demonstrates that Flex is being used to develop very powerful, interactive user interfaces. It's more than eye candy. It enables Pearson to present a powerful case for licensing their software.
That's not to say that I'm sold on Flex, but if Pearson is using it, it's going to gain a lot of traction.