Hi,

I'm curious what the 'usual' method is of attaching roof trusses to an ICF wall?

I went up and looked the other day (roof in garage is open, and has opening to whole house aswell), and the trusses just seem to be nailed in (with nails diagonally) to a long 2x8 all along the house perimeter..

There are many ways to attach trusses to ICF.  In this part of the country, i.e. South East I have used several methods. 
On the coast I have used a Simpson Hurricane truss attachment embedded in the top of the ICF wall..  Have also attached trusses on a double top plate that is attached to the ICF wall with j bolts.  Then using Simpson hurricane ties fastened to the top plate.  Also have done the same on a single top plate.  Sometimes the top plate is impeded into the ICF form again with j bolts.  I am sure there are other methods that I have not covered. 
Sometimes the attachment method will be dictated by the engineer or building department. 

Up here in Minnesota we have been using the simpson LTA1 and have been very happy with the results.