PG: Make the decision on what sites to show subdomains on at page render time, based on a flat file list of domains. Have a job run every now and then that checks submitted urls, and creates the flat file list of domains, including any domain with more than N different subdomains submitted, where N is high (maybe 400 different sub domains or something). If this file would be large, just build it based on the last M submitted urls (nobody will be browsing the older ones anyway, so if a domain comes and goes oh well).