With the advent of Intel-powered Macs, virtualisation solutions exist for running Windows in a virtual machine under Mac OS X 10.4 and newer on Intel Macs. CPN Tools can be brought to run in this environment with reasonable performance for day-to-day use.
The way I use is to run CPN Tools under Windows under either Parallels Desktop (www.parallels.com) or VMWare Fusion (www.vmware.com). You may also have luck with Sun Virtualbox (www.virtualbox.org), though we do not test that! The programs emulate a virtual machine, and you are thus running the Windows version. The programs have an option, which allows you to hide the Windows desktop, so it seems like you are running Windows applications (check their web-pages). Parallels and VMWare allow you to test the program for two weeks before purchasing, and Virtualbox is free. I would suggest installing 7 in the VM. Note that these programs are Intel only, and it is not advisable to run them with less than 2 GiB of RAM on your computer.
A good configuration as of February 2013 is Windows 7 32 bit in a virtual machine with 1 GB of memory, and in fact all development of CPN Tools is done in this way. You can run the simulator natively on your Mac and have the Windows GUI use that, which is useful for heavy-operations, such as state-space analysis and simulation-based performance analysis.
CPN Tools has a problem if you are using the graphics adapter provided with Parallels for accelerated graphics (and sometimes VMWare). You can run CPN Tools using safe mode as supported by version 3.0.2 onwards, which will solve this. It is possible to convert a VM created with one of the tools to a VM for the other tools, so you do not have to install Windows twice to test both tools. You cannot keep changing between virtual machines, however, since Windows usually requires re-activation after the conversion.
If you are using Time Machine, it is highly advisable to turn off backup of the virtual machines, as they change each time you launch them, causing a backup of several giga-bytes each time you launch CPN Tools. My Parallels Installation is at