The December update (codenamed "Polaris") began rolling out to early-access program participants in mid-December 2012. This week marks the beginning of the rollout for new and existing customers. Users can choose whether or not to apply the update, using the opt-in model.
The December update is optimized for Dynamics CRM Online users, but many of the same features are available to on-premises customers via Dynamics CRM 2011 Update Rollup 12, officials said.
Update (Jan. 29): As a few readers have mentioned, Microsoft pulled Update Rollup 12 and has not issued a replacement for it. Not sure when to expect it to be available again, at this point.
Update No. 2 (January 31): Micorsoft has republished and revamped its Update Rollup 12.
Early last year, Microsoft officials said they'd have CRM apps for iPad, iPhone, Android and Windows Phone available in Q2 2012. The coming Dynamics CRM mobile clients/service were set to start at $30 per user/per month, with support for use on up to three devices per user.