PR -1 used to mean the kind of PR 0 that came from a penalty or ban, in contrast to the kind of PR 0 that came from not yet showing enough links to get up to PR 1. Could this be what you are referring to? I haven't heard anyone talking about that for a long while.
Pixeltierra, I tried a couple of domains on the tool you mentioned, (like webmasterworld.com for instance) and it also gave -1/10 for them. So I would suggest that the tool is perhaps just broken/rubbish.
How is this 'confirmed' in the Webmaster Tools exactly?