Formatting for dates in the wiki is as follows:
day month year (ie: 19 November 2006)

For BETA items I have put a retire date of 27 June 2005 (the day beta ended).

What should we do about release dates for items that were in beta but are still around? There are two possible dates:
23 May 2005 (Beta Start)
27 June 2005 (Official Opening)

Thoughts about which to put?

I think the May date would be good :)

I'm inclined to go with the start of beta. (possibly link it to the page on the subject?) Or even "from the beginning".

I think you should put the non-retired beta items still under the beta checklist, and just put that it has not been retired yet.