Chris, that's an interesting issue. The provider is issuing a 301 redirect to a-corvette-christmas.aspx/+.aspx as that is what the friendly Url provider is calculating as the correct friendly Url for the page.
I think the problem lies in the fact that when the friendly Url provider is called with a 'replacement' page name (in your case, obviously 'a-corvette-christmas.aspx' instead of default.aspx, it goes on the end of the Url OK (which is why it is generating OK).
The problem comes from working out what the Url should have been, given the old version. To get the 301 destination, the friendly Url provider is called again, with the details of the page, only I think it is making the assumption that the page used default.aspx as the ending, not 'a-corvette-christmas.aspx'. The friendly url provider is incorrectly thinking that 'a-corvette-christmas' is instead an incomplete part of a query string, hence it is filling in a blank space '+' as the second part of the query string. The same thing actually happens to my site if you request the 'products' page with a .aspx on the end - in incorrectly parses the query string, thinking the page name is half a querystring identifier.
In short (if all that doesn't make sense) you've uncovered a bug. I'll have to put my thinking cap on to fix this one though. There may not be a satisfactory solution as there is no way for the provider to know when a module on the page is using a non-default page name, and thus it is hard to detect. I'll have a shot at it though, some regex fancy work might do the job.
On another note - is the tagging system on that site part of the engage publish module?
It works OK, but I bet you can't tag all your site content like you can with my Tagger module :)
[shameless self promotion - how else am I to get my own z06?? (not that you can buy them here anyway)]