The c++ code that interfaces with the USRP and daughterboards lies under usrp/host/lib/legacy. The name “legacy” was added to distinguish from the (at the time under development, now apparently abandoned) inband code. A couple of references:

Update 22.08.2009: Not anymore in trunk since 08.07.2009: It was moved from usrp/host/lib/legacy to just usrp/host/lib