The short skinny:
- you can use it as a standard USB Mass Storage device on any operating system (Win/Linux/OSX) without installing anything; just stick your 2gb microSD in, format it and use the USB cable.
- if you want to *charge* your BlackBerry as well over USB, you'll at least need the Device Manager installed (a subset of the Desktop Software) which adds the custom driver for BlackBerry devices.
For the second one, you can download a small 12meg installer of just the Device Manager, and keep it on your microSD card!
That way you have it if you ever need to install to get the charging functionality on the go with a new PC.
Go here: BlackBerry - BlackBerry Desktop Software | Applications to Synchronize Your BlackBerry and Desktop Data
Select the download option for the 4.2 software (it takes you through 2 more screens I think), and then scroll down and look for this:
BlackBerry Device Manager v4.2 (English)
* Download includes: Device Manager
* File name: BlackBerry Device Manager v4.2 (English).msi
* File size: 12.0 MB
* Date posted: 10-Oct-06
That's the mini-installer to grab and save on microSD.