Its all there and sent up each time you communicate with the Market, there are strings like "T-Mobile" and the model etc sent in the clear, I know because I sniffed a session with wireshark a while back. But I didn't see the call to actually get the info in java.
One way would be to reverse engineer the Vendor.apk. This isn't as bad as it sounds - the ddx code is almost readable, it explodes each method in its own format. Look here
strazzere.com/blog/?tag=vendingapk, download Vending.rar - thats the output from DeDexter, a tool to decompose .dex files.
Google use a construct called a ProtocolBuf to comminicate over http - its ultra efficient and documented here:
code.google.com/apis/protocolbuffers/docs/overview.html
So if you look in android/vending/api/RequestDispatcher.ddx you'll see its basically populating a ProtocolBuf instance ready to send up, and the fields in there are as I say the terminal name etc. You'll have to hunt round those files to find the actual calls, usually the full package name is specified too so you might get lucky.