I think Wondertouch had announced their plans for the move into AE long ago.
It makes a lot of sense, the particle system available for AE are very comprehensive, expensive and not too easy to fathom. They do bring some nice feature though, like having the emitters light up other objects e.g. text or having other light sources light the particles in a 3D space.
10,000 users sounds like a lot, until you work out how much each one of them has to spend per year with your company to pay the running costs. I think that's why there's been a lot of acquisitions recently in their marketplace.
if all 10,000 users bought the latest version that's $3,890,000 in revenue. 3.0 for Mac came out in March 05, so that's under a million a year spread out, @ 20 employees that's an average of ~$50k a year.
So either they made a LOT on emitter sales or it was only a matter of time...
Either way PI is an awesome program. Would love it as a plugin to Vegas.