Achievements on the Xbox part of the Win Phone 7 definitely go into your regular gamer score. See the Gizmodo piece here.

Quote: Every Xbox Live game on Windows Phone 7 has a 200 gamerscore, just like Xbox Live Arcade games, and will accrue to your overall gamerscore. That integrated experience is reflected in your Avatar as well: any customizations you make on the phone will be reflected immediately on the console, and vice versa.

Pretty cool.

That said, I'm waiting a year to see how things work out with the phone OS in general. I just bought an android phone last week (EVO).