I'd like to control a unipolar stepper motor through L298N. I've this board http://www.seeedstudio.com/depot/l298-dual-hbridge-motor-driver-p-284.html. How do I hook them up?
I don't think it's a good idea. Better is to use a Darlington array like a ULN2803, or just even discrete transistors.
In fact, the main advantage of unipolar windings is that they are easier (?) to control and don't need the complexity of an H-bridge. Why do you want to re-introduce the complexity of an H-bridge?
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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons
I now remeber I bought this board to use with a bipolar mottor. I just noticed I'll need to control a unipolar motor though. 
GedgetShield does not really fit the porpuse in my case - I need a more specialized piece of hardware, just like that board I pointed out as I'm going to deploy it in a dedicated machine. Recomendations?
Side question: I read a blog post (sorry, cannot find reference anymore) one may use a unipolar stepper as a bipolar one. What are the pros and cons of doing so?
The pros of bipolar drive are higher torque. In unipolar drive you are really only energizing half the winding at a time. The cons are that you need an H-bridge, rather than just a handful of transistors, but you already have one 
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The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected
Hi,
How many wires from your motor?? 5 and you're stuck with Unipolar. But some have 6 and you can use them as Bipolar, connecting the whole winding. (Your desired supply voltage may be higher then)..
But no problem with using the typical H-bridge for Unipolar; you just use 2 connections per motor and the driver is only sinking current, never sourcing.