Our trusted first cousins at Lifehacker were able to download a copy of the publicly available Beta 2 of Windows Vista last weekend, and in a fine how-to article show you how to dual boot it with Windows XP. How they were ever able to download the file from the zoo that is the Vista download area, we'll never know. Even after repeated attempts using a variety of download managers, we had to just punt and order the DVD.
Anyway, the industrious and risk-taking Lifehackers used a copy of PartitionMagic to create a separate primary partition for Vista, and then went ahead with the install. All went well and the new OS was configured without incident, and now at boot-up, there's a choice between either Windows XP or Windows Vista.
If you're not crazy about the idea of fooling around with Beta software, Lifehacker has an upcoming series of articles through which you can live the Vista life vicariously.
How to dual-boot Windows XP and Windows Vista [Lifehacker]