I certainly wish we had sorted some of the beta threads into their own category, and we certainly may be able to add a prefix to such threads by using the date of the original post. We do not traditionally close any old threads, as the forums are designed for free and open discussion - no matter how old any discussion may be. This allows for maximum flexibility in many cases, but could cause confusion in others. However, changing the information this late into the situation is something we will discuss soon (admin-side). To determine the necessity. I do not think we have many active beta threads, but I know which threads you are talking about: i.e. the "does not work with Windows 7" related topics. In these instances, it is best advised that members are aware of the dates of each post and the RTM date of Windows 7. People were using the site well before the release of Windows 7.
It is nice to see an old thread still open and being discussed though, even if some people are confused. This leads room to explain that the OP has become out of date, and opens room for a renewed discussion of the original material. One thing I do not want to do is start editing old threads other people wrote unless absolutely necessary. This is a good point, Highwayman, and we will look into this.