Hi Adam,
As I understand it what should be happening is that you arrive at the Details page only by clicking on a link from the Month/Week/Day/List view pages, which always populate the ItemID, (and SelectedDate as well). Therefore you should never arrive at the details page without an ItemID. If you do, (maybe from somebody guessing URLs), then it should redirect you (via NavigateURL) back to the basic events page. The URL should look something like:-
http://www.domainname.com/PageName/tabid/54/ctl/Details/Mid/862/ItemID/237/Default.aspx?selecteddate=23/08/2008
This is derived from a page on my site which has DNN FriendlyURLs switched on (the latest version from 4.8.x I think, because they upgraded it a couple of version ago and you have to tweak web.config).
When you call the base page, it should deliver the Month/List/Week views. This is defined in Events.ascx.vb, and is taken from a cookie on your machine or the module settings. There should be no way it starts of on a Detail page unless you have entered that URL or have linked from one of the others. Therefore it would seem URLMaster is doing something strange with the URLs, I just don't know what.
For my part I could potentially have a look at this, but I don't have a copy of URLMaster. Also I'm buried in developing some stuff for 4.1.0, so don't want to divert from that for a couple of weeks until I get it worked through.
I'm quite happy to provide some help via the forum for the time being though.
Cheers
Roger