I'm no fan of beta software. Defensive Computing means never dealing with new software, be it in beta form or shortly after a major upgrade. Experimenters and the curious should use virtual machines.
Now, from Sunbelt Software comes this warning about the beta version of Internet Explorer 8 and Windows XP SP3. In a nutshell, if you install the first beta version of IE8, then upgrade Windows XP from SP2 to SP3, then upgrade to the second beta version of IE8, you are now married to both IE8 and XP SP3.
You can see the original blog posting from Microsoft or, better yet, avoid beta software as a rule.