A lot of users here have had problems with their mobile gaming recently. Achievements unlocked regularly during gameplay on the Wp7, but would not synch with Xbox LIVE. The Xbox Support Team is aware of the issue and is working to fix it. Here is their statement.
Hi all! Thanks for your reports of the Achievement issue on your Windows Phone 7 devices. We've identified the cause of this issue and we are working to resolve it asap! We should have a fix in place today, but the fix will be finalized within the next 24 hours. In the meantime, thank you so much for your patience! Please let us know if this issue gets resolved for you within the next 24 hours.
Thanks to all the community members that sent this in. Don't forget to let us know if your achievements start registering properly again! Happy achievement hunting to you all.