While the factory does not officially support racing Boxsters (harrumph), some free spirits are working on it.
from ebay motors, racing cars tab, filter for "road racers":
http://cgi.ebay.com/ebaymotors/GAC-Koni-Challenge-SCCA-NASA-PCA-Boxter_W0QQitemZ270101495943QQcategoryZ98064QQssPa geNameZWDVWQQrdZ1QQcmdZViewItem