I only have some older pre-release guitars here, but after one completely died for strumming, and the other became flaky on downstrums, I cracked them open to see if I could perform surgery. I'm not sure if the final guitars use the same mechanism as these or not.
The strum bar actually has two sensors to detect up and downstrums seperately (that's how they have the upstrumming-only achievement for bass). However, on my guitars, I'm definitely not a fan of the mechanism they used. The strum bar, in each direction, pushes a piece of plastic attached to a metal bar in, connecting it to another, central metal plate, completing a circuit. On my totally broken guitar, both plastic pieces had snapped off (and trying to jury-rig a glue-based fix was an epic failure), and on the flaky one, I can't seem to see what the problem is (it seems to be connecting fine, but registers anywhere from 0-3 'hits' with each downstrum).
Anyone else crack theirs open to confirm how the mechanism works?