It's an intriguing syntax. One thought would be to offer both styles of Hamlet with different function names. However, that runs the risk of creating two different Yesod communities. Obviously, switching syntax entirely will completely break old Yesod applications, but especially with external Hamlet templates it should be a piece of cake to write a converter.
Overall, I like, though I haven't spent a huge amount of time thinking about it. I think it would be good to get broader community input on this; you think you could send an email to the web-devel list either linking to this page or copying its content?