EW's Michael Ausiello has a bit of dirt on this -
"Exec producer Carter Bays isn't saying either way, but A Mother insider tells me that she is not the mom. Still, it looks like Chalke will be sticking around next season -- in some capacity. "We haven't talked to her or her people about it yet, but there's something very exciting about next season [involving Stella] that will sort of be alluded to in next week's episode," teases Bays. "We're excited to see how it'll play out next season." Speaking of Monday's ep, Bays says it ranks among the show's best. "It's a real good, classic How I Met Your Mother," he declares. "There are a few episodes of this show that, when we were shooting it, felt like an old movie. You just want to put it in black and white. Next week's episode felt like one of those."

WHAT IN HOLY HELL JUST HAPPENED

Oh my god, Danny DeVito, I love your work!

I'm in love with these two.

Just watched the first episode. You've got me hooked.

I'll have to check this out, I love the cast.

JD, as much as I've seen you mention Chuck in the past I'm kind of surprised I haven't heard from you on it recently. There's a big campaign going on to get it a third season since it's in danger of getting canceled.

I marked out for the Buffy reference.

Of course Scooter was the waiter! He said he was gonna be the best waiter ever!

I loved the episode, by the way.

Not unless future Ted finds out, such as in the flash forwards to their college reunion or LAME's envelope-opening stuff.

My mind, it is blown. I can't wait to see what they have in store for this.