What are some good exercises that an intermediate/advanced VB.NET web programmer should to do gain syntax chops on C#?
I imagine some good examples would be:
Play with your own code using an automatic converter.
It should work fairly well for most things. You'll mostly just need to figure out how to rework lambdas and some other situations like that.
Reflector is also very good to help you figure out how to play with converting your code.
As for learning the differences, see MSDN's white paper on this. It's a bit out of date, but a good starting point. A more complete, but less textual reference-like comparison, is available here.