Here is a podcast describing VLAN hopping. While VLANs are touted as a VoIP security mechanism, in reality they are very easy to defeat and are misconfigured more often than people realize. The podcast describes a tool/technique for VLAN hopping. We commonly do the same thing as part of our VoIP security assessments. In a typical network environment, it is quite easy to configure a laptop/PC to get on to/hop on to, the voice VLAN.
The following article (also in my typelists) offers a very negative view on VoIP in general and especially VoIP security. The article also offers 10 tough questions that you ask your vendor. The author states that VoIP systems are not secure out of the box, which for the top-tier vendors, really isn't the case. The author also states that it just isn't possible to secure VoIP, which I also don't agree with - its possible, it just takes some work, monitoring, and in some cases, some additional products and features.