Anders Wallin (from CNC Zone) built an upgraded mill spindle with one of those a while back. He's geared it at 3000rpm and 6000rpm for finishing work! His blog is well worth a read. I found a couple of posts where he discusses the build of the new spindle:

http://www.anderswallin.net/2006/10/...opti-cnc-mill/
http://www.anderswallin.net/2006/12/...dle-installed/