I know who the mother is. Or do I?
In the most recent episode, they mention a website. It is http://tedmosbyisajerk.com
On that site, there is a 20 minute song by the person who Barney slept with while pretending to be Ted Mosby, Architect, where she just bitches about how much she hates (and loves) Ted Mosby and how she thinks he should die (or call her back). At the very end of the song, there appeared to be backwards speaking. I downloaded the song, and reversed it, and...wait for it...THEY REVEAL WHO THE MOTHER IS!
I don't know if I should post this information here, lest i give away the answer to those who don't want to know, but if you do, i'll post a link to the audio file here:
Select the following text to see what it says:
So there you have it. You can stop watching the show. Or can you?