Unfortunately, though, development of Script# has since then all but ceased, with hardly any compiler features being added over the past 3 or so years. When .net 3.5 was new, not having implicitly typed variables or lambda expressions (or quite a lot of other features) wasn't too bad, but today it feels very old. Once again, server-side programming is an order of magnitude less annoying than client-side programming.
But no more. About a month ago I released the Saltarelle compiler. It is intended as an almost drop-in replacement for Script#, but with almost full support for C# 4. And it is also open source! Over the past weeks it has matured to the point that it is now stable and ready for public use. There is a Getting started guide for those new to the concept, and also a guide for migrating from Script#.
I do not think that this is a tool that is suitable for everything but if you are developing a fat client application in HTML (especially, but not only, if your serverside code is .net) it will make your life easier.