First please listen to this extract from Richard Thompson's "Johnny's Far Away"
http://www.box.net/shared/c2411mk6rxzad0s3n85p
Now this is interesting to me because I can count this two ways. There's a clear 4 beat pattern, but within that there is a faster 3 beat pattern.
Now, OK, one thing that occured to me was that it is in 4-4 but there are an awful lot of triplets being played. But if you used triplets that much wouldn't you want to write the piece out in something other than 4-4 so you don't have to note all those triplets, and wouldn't that then mean that the time signature is modified?
Also if you listen to the guitar and bass solos they seem to stick within that "3 inside of 4" timing, but when the fiddle comes in after the bass solo it seems not to - that makes sense in the 4-4 context and seems to ignore the fast triple beat.
So what time signature is this in? And is there something polyrhythmic going on here?