Probably to late for this Beta since it has been on for a while and
release will probably be the early part of 2008.
If you may have an interest in a future Beta, create a Beta profile:
http://connect.microsoft.com/
See what connections are available.
When you find one or more you will be seriously interested in, apply.
That way you can become known to the Beta community and as you are
experienced, more opportunities may be offered.
If you may have an interest in Beta Testing Windows 7 in a few years,
start now.
Once the Beta is starting, your chances for being selected are already
slim as they are for you and the SP-1 Beta at this time.