I had some tests with Mac VICE and GuruTerm.
If I use the inernal ethernet of my MBP (here en0) and connect to my router with a cable then everything works as expected. GuruTerm's DHCP works and later on I can successfully 'dial' and log into a BBS...
If I use the airport WLAN card of my MBP (here en1) then it doesn't work. The symptons here are: the airport connection drops immediately and VICE cannot open the adapter. Some googl'ing told me that the libpcap library which is used for VICE has a bug concering airports and only the most current CVS version of libpcap should work. Unfortunately VICE is compiled against the latest stable version of the lib and still has the bug.
I recompiled VICE with a fresh CVS libpcap and now the airport stays connected if I use en1 for RRNet. Unfortunately, the packet injection still does not work as expected (maybe another bug in pcap?) and so I still can't connect via en1

I will have a deeper look at that later...
Since other programs using libpcap, e.g. Wireshark, tcpdump... suffer from the same problems on Airports I hope the problem gets fixed soon...
So stay tuned...