Thanks Sebastian,
I gave that a look and both sites (fresh install and the newly upgraded 'broken' site) have this under Host > Host Settings > Advanced Settings > Friendly URL Settings:
Use Friendly URLs? Checked
.*DesktopDefault.aspx(.*) --> ~/Default.aspx$1
.*EditModule.aspx(.*) --> ~/Default.aspx$1
.*/TabId/(\d+)(.*)/Logoff.aspx --> ~/Admin/Security/Logoff.aspx?tabid=$1
.*/TabId/(\d+)(.*)/rss.aspx --> ~/rss.aspx?TabId=$1
[^?]*/TabId/(\d+)(.*) --> ~/Default.aspx?TabId=$1
Any other places I may be missing?
Also, this occurs to me as well: Recently I posted about a problem I was having with the Login control of another site. The problem was that the design had called for a "Login" page to be listed in the Menu / Page structure. When I added the page, I had to put the Account Login module onto it because whenever the control was clicked, it would load the page and not the 'default' Login page (the fact that the URL wound up being domain.com/login.aspx was no doubt causing the site to flip out). Well, the client changed their mind and asked me to remove the login page and just use the control link. No big deal I thought, and removed the page but then the login control link wouldn't work, it just clicked like a dead/null link. I followed the several steps I found online from Mitch Sellers' OpenSesame.aspx file and even tried manually editing the DB (same thing the ASPX file does). Nothing worked, so I finally just killed the site, setup a new DB and reinstalled.
The point of that paragraph was this: I believe at one point, the site that is having Sitemap problems used to have a Sitemap page which was removed during the 'update'. Now I'm assuming this is a similar issue of having a page name that reflects one of the 'core' .aspx pages.
Thoughts, or have I just confused the matter more?
Edit: I took a peek in the DB Tables>dbo.Portals>Columns section (where the LoginTabId is) but I'm not seeing one for Sitemap, and as I'm more on front-end development, I'm not going to go poking this bear.