Checkmate

Is this checkmate?

Not if black is going up.

I'm assuming that black thinks it checkmated white with the pawn move, but white counters by en passant capture to counter the check with a checkmate of his/her own!!!

It is. What we cannot determine is if it's white or black who checkmates.

My diagrams always have White on bottom.

It is if Black's last move was b6-b5. 

However, if Black's last move was b7-b5 then it is not mate, since, as Jion_Wansu pointed out, White can capture the b5 pawn with his a-pawn or his c-pawn, and mating Black in the process.

Hence, since this information isn't stated in the problem, we don't know if this is checkmate or not.

White is forced to checkmate Black next move.  It is an anti-checkmate.