One of my fellow engineers 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.
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).