Here is Haron Yahya's site on the return of Prophet Esa (SAW) and the emergence of the Mahdi. I'm sure many of you have seen this site before:
The site gives some great evidence, which I believe to be sound, that most, if not all of the signs regarding the coming of the Mahdi have been fulfilled.
But if you read through the site, you get the idea that since the signs have been fulfilled, not only should he already be among us - he should also be fighting at this very moment.
So in that case, who is the Mahdi?