Straight away i would say "put the SM57 on the edge of one of the speakerelements, very close (<1")"
But of course, you have to experiment, I'v used a cheap BIG chinese condenser mic on these guitaramps and gotten a GREATE sound.

I agree - '57 straight at the cone just a little in from the edge. A condensor about 10 - 12 inches away can be nice (watch for phasing problems). play with the panning and levels for a sound you like.
If you are recording the amp by itself you can try a room mike as well (8-10 feet away).

Thanks alot for the usefull feedback.

Just one more thing, i have a phase inverter switch on my samson mic preamp, which mic should i invert the phase of (the condenser or the SM57)?

thanks alot

You will only have to invert the phase if you are having cancellation or phasing issues.

If you do need to invert one of the signals it doesnt matter which one it is.