The Internet Storm Center (ISC) is offering a free Registry Search Tool. This neat new tool will locate the registry key values greater than 255 characters in length.
Windows Registry - Nasty Games of Hide & Seek
ISC Registry Search tool -- locates long key values
We have started to see some possible reports of malware which utilizes this concealment technique in the wild. Products that have been reported to be able to query/report/delete/etc these keys:
AppSense Environment Manager
HiJackThis v1.99.1 (SCAN function)
HiJackThis v1.99.2 (in development)
Sysinternals Autoruns (mixed reports)