The general rule of thumb is to host the VM on a separate drive than the host. Is this necessary with an SSD drive? I'm looking to get a 120gb SSD for my mbp to run Windows 7 VM's as well as OS X. Any other tips are appreciated.
SSds get their performance from a lot of internal parallelism. But in order to make use of it they need a relatively heavy IO load. On desktops most good SSDs are absolutely starved for IO load. Adding a VM will not hurt performance at all. At least not until you start moving enough data to saturate the SATA connection.