One of the things engineers are going to have to think about when the new widgets api's come out is not just to think about the device, but rather to think about the server side. I mean how great is your widget going to be if your server is being hosed by millions of clients.

The biggest problem I believe is going to be dealing with scalability issues on the server side as there are plenty more phones than laptops and with wifi being cheap and data plans becoming cheaper there could be a whole load of problems dealing with orders of magnitude more connections than a regular windows based clients.

That why one of my favourite feeds is the high scalability website where they cover how the really big sites on the web work and scale.
Recently they have covered the architectures for flickr, youtube, digg and typepad.