Štefan, one of our CCIEs, was working with a customer who wanted to have UMTS backup for his primary link. They’ve got it to work, including IPSec+GRE running over UMTS to provide the necessary privacy/security. However, it turned out that all wireless solutions (at least as offered by nearby Service Providers) are ridden with ridiculous round-trip times. For example, UMTS transmission delay is equivalent to the time it takes to send an IP packet from Central Europe to US West coast.
Štefan’s conclusions were modest: obviously UMTS technology is not ready to provide backup for VoIP traffic (which prompts an interesting question: how do the Service Providers plan to provide reasonable quality IP-based voice calls over UTMS?). The reality is way worse than that: a few hundred millisecond round-trip time can kill web browsing performance if you have awfully-designed web sites (and a lot of intranets are not well-designed because the developers never considered the delay impact).