They wouldn’t tell, I guess… I don’t think it’s custom built (but who knows). If not, then it might be a clone of Oracle. Only thoughts…
I posted about this here the other week but.. yeah it's GFS (Google File System) with master nodes and chunkservers. The chunkservers hold the bits of data and the master nodes hold a metadata index of the chunkservers. MapReduce forms the backbone of the framework for how queries get handled on the hardware. If i recall, CNET had a good article on it last year. Edit: Here it is, well worth the read http://news.cnet.com/8301-10784_3-9955184-7.html
I phoned Google customer service about this question, they said they are using MS Access 2007 now and they are busy negotiate with Microsoft for MS Access 2009 pre-release. It looks like them business really start up.