I am a .NET webdev using ASP.NET, C# etc... I \"learned\" javascript in college 5+ years ago and can do basic jobs with it. But I wonder if it is useful to become proficient in
If you are a webdev then yes, you should be proficient with Javascript. Javascript is a major part of making web apps as interactive as desktop apps.
With that being said, learn to use one of the cross-browser compatible libraries like JQuery, Prototype, etc. We do not need to have any more single browser crud created using Javascript, just because any real man/woman rolls their own.
A few things to learn in Javascript: 1. Basic syntax 2. The various flavours of function declaration. 3. Passing functions around and how to use passed in functions.
I don't think a lot of deliberate learning makes sense (but of course you need some basic knowledge), but I also think after some years of web development you'll become pretty proficient in the language anyway :)