I ditched my Airtel modem recently, been pairing my Samsung S5570 Galaxy to my laptop and using the data off of that since. It is an easy convenient option.
Was asked about doing the same on a Blackberry 9780 and below are the instructions to create a wireless network from your blackberry and using it as a modem. Devices connected to that network can surf the internet through it.
Enabling Mobile Hotspot
From the homescreen of the BlackBerry navigate to Manage Connections > enable Wi-Fi > enable Mobile Hotspot
Once the Mobile Hotspot has been enabled, by default it will ask you to set a password to secure the network.
And that is all it takes, you should be browsing in no time.
There is another option to connect the device to the phone via the USB and browse with that but it is much too lengthy. However click here are the full instructions courtesy of blackberry support