[cadwr]

m.k. has released VBA8, a Gameboy Advance emulator, for Windows Phone 8. Unlike the free SNES8X, VBA8 is currently $3, though there is a trial. I'm not sure what the trial restrictions are.
So far it is running Advance Wars really well; I've yet to test any other games. I'm currently using the trial, but will grab the full version later today after some more testing.

US Store link: VBA8 | Windows Phone Apps+Games Store (United States)

edit:
The feature set is the same as SNES8X- save states, skydrive integration, .GBA association so you can import roms from email or web.
edit edit:
I should also mention that I'm at work, so I can't test sound. If anyone tries it out in a better environment, let us know how sound is.
more edit:
Save states are one of the trial limitations. Also, it appears m.k. forgot to change something in the code. If you click the Buy Full Version button when you try to do a save state it brings you to SNES8X in the marketplace.

[Keith Wallace]

So does that mean NO saves in the trial?

[cadwr]

You can do regular in-game saves as far as I know, just not save states.

[dashimoto]

Oh maaaaan hopefully there will be a WP7.8 version

[busngabb]

Awesome news. There are no other gba games are there, Advance Wars is the only game there was 😉

[Mister Gore]

The trial seems limited to 2 games, and no import/export of save states.

Every game I've thrown at it so far runs beautifully. Already made the purchase.

[cadwr]

Yeah I ended up buying it yesterday, everything is running really well. At this point the only thing I would like to see from this and SNES8X is cheat support. Well, and Bluetooth controller support would be nice too.

[Andrew Milne]

Advanced Wars is one of my favorite games of all time. Looking forward to fireing this up again!

[Zeroplanetz]

Could I try my own rom in the trial?