Why does the useragent include this information? I can't see why a website would need to know.
For example:
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/534.24 (KHTML, like Gecko) Ubuntu/11.04 Chromium/11.0.696.28 Chrome/11.0.696.28 Safari/534.24
Both Firefox and Chromium on Ubuntu do this.