I'm considering building a framework for VB.NET, and using the My namespace to plug it into VB seems like a reasonable idea. Is it?
If it fits, by all means, use it. Since you didn't offer any further information about your framework it's hard to say. I wouldn't put general-purpose stuff into the My namespace (such as the My.Computer stuff) because there isn't really any advantage to putting it there. However, application-centered helpers fit in well.
