To use Airtel blackberry as a softmodem. Follow the howto or here for linux using xmblackberry.
The modem initialization parameters for airtel are
+CGDCONT=1,"IP","airtelgprs.com" and the number to dial is *99# and no username and password is needed.
I am able to use ssh though there is high latency but haven't been able to use PPTP based VPN which runs on protocol 47(GRE) over IP directly, although I suspect an SSL based VPN might work.

Update:
Yes you need to install the Desktop Manager as well and make sure you can access your blackberry from the Desktop Manager.

Airtel GSM network for pre-paid network in Pune just stopped working about half an hour ago, without any warning of any sort. Don't know if it happened nationwide. Suddenly you hear first level support from airtel contact centers claiming 'server is down'. If the server is down get a redundant server online dudes. Airtel also happens to be most persistent SMS/Phone spammer around in India. Repeated calls asking them to stop sending un-solicited SMS'es are met on deaf ears. Didn't TRAI fail in its duty as a regulator in not fostering a free-market environment by not implementing number portability. As a side note I am an un-happy corporate Airtel Blackberry user. Extremely bad network policies, they not only use an HTTP proxy, the proxy for India is sitting far away at UK. TCP/IP doesn't work at all, so no ssh and Airtel customer support claims that the device can't operate as a modem for a PC ( probably because they want to sell datacards as a separate product ).

Update 13-OCT-2007:
TCP/IP works after specifying APN: airtelgprs.com and to use airtel blackberry as a soft-modem from your PC follow the links here
Recently the speeds on airtel blackberry have improved(maybe its better in Bengaluru) and are a bit more than a typical dial-up connection from BSNL but still not usable enough for VPN'ing to office.