Well, it wasn’t much of an accident. I took a domain that was several years old, and a PR3, but I hadn’t done much with it for months. The domain was to expire on 19th of January 2011. I let it. A few days later the site stopped resolving and was replaced by a parked page.
On the 28th of February I was sent the message that the domain was to be deleted that day. I still waited and on the 1st of March I renewed the domain.
As of the 3rd of March Google reported the domain as grey PR (using the SEO Book toolbar) and no cache date. Searching for the domain name (which is a unique phrase) and my domain wasn’t returned.
Now it’s the end of the month, and I’ve searched again. The domain is returned as the second result and the Page Rank now appears as PR1 with a cache date of 3 weeks ago.
The domain remained a PR3 after the January Google Page Rank update so the drop unfortunate and looks like it was recalculated after the expiry. As regards the cache, as the domain still gets no content love from me so the date is reasonable.