Our friends over at MacTech have taken on the laborious task of running Parallels Desktop and VMware Fusion, comparing them against each other, running both Windows XP and Windows Vista, against Apple's Boot Camp, and against a standard PC laptop.
MacTech's tests included real-world activities in each of the main Microsoft Office 2007 applications (Outlook, Word, Excel, and PowerPoint), along with tests for network and filesystem I/O, Internet Explorer, and cross-platform tasks that involve working with the host operating system (like viewing a PDF attachment to an Outlook email message in Apple's Preview).
The full MacTech article makes for a fascinating read, but it seems to boil down to the following conclusions.
Keep in mind that these conclusions are relevant only for the things MacTech tested, which did not include gaming (where Boot Camp probably has the edge over both virtualization options) or applications that can use multiple processors (where VMware Fusion would probably outperform Parallels Desktop).
If you want to analyze MacTech's results further, you can download an Excel spreadsheet containing all the test data.