I've recently generated a sitemap, but Google has it flagged as invalid.

Line: 20   
Invalid tag value
This XML tag has an invalid value. Please fix it and resubmit.   
Parent tag: url
Tag: priority
Value: o.8
Problem detected on: Mar 11, 2010

Sitemap URL: [ External links are visible to forum administrators only ]

Line 20 reads:  <priority>o.8</priority>

I have no idea what is wrong with it. Any value up to 1.0 should be proper, right? It certainly had no trouble with a <priority>0.85</priority> not far above. What's wrong?

did you set "individual attribute" for that URL? Looks ike first character is a letter "o", not a zero "0".
Good eye. It was hard to notice with the the default font. :)