Among these service nodes there are "transport" nodes, like for example jabber.org.by, that provide you with gateways to other IM systems such as ICQ and MSN.
So now we are getting closer to the main point. In order to use ICQ and MSN via GTalk web interface or standalone program, do the following:
- Download and install a good jabber client. I used http://gajim.org/.
- Run it, connect to GTalk server by supplying your gmail username/password and gmail.com as the jabber server.
- Discover services of jabber.org.by, in gajim it is done by selecting Action->Discover services, entering jabber.org.by and clicking Go. You may find and use any other transport server but I've been told that this one is one of the most stable ones.
- Subscribe to MSN and ICQ services supplying your login credentials when asked.
- Quit jabber client.
Note: gmail web interface will display a contact chat invitation for each of your ICQ/MSN contacts. Just ignore that - the dialog will disappear by itself.
Many thanks to Rinat and svyatogor for opening my eyes!