As Dieter has recently discovered, the nanoBTS has an optional serial port. You need to solder two small bridges on your nanoBTS PCB and then you will get a RS232 port (at 3.3V) to the embedded PowerPC.
On this serial port, you can use an extensive command line interface (CLI) to display the status of the BTS, and for any kind of interactive debugging.
I only wish they had made that interface also available via TCP/IP :) Not many people will want to risk soldering their nanoBTS and thus loosing their warranty... it's not a cheap device, after all.
A description of the pin-out, including a picture for which solder bridges you have to set can be accessed in the OpenBSC wiki