According to Gerard Berthet of mibCentral, the domain registration expired and was quickly snatched up by someone trying to sell the domain back to him. As a result, mibCentral is now operating under a new (domain) name, MIBDepot.com. You can find the site here:

http://www.MIBDepot.com/index.shtml

MIBDepot (formerly mibCentral) is a free service providing a searchable repository of over 5000 SNMP Enterprise-Specific MIBs from many different companies.