rowlands appears to be correct, though the behavior only seems to occur with IE7 on my home desktop (XP Home SP3) but not on my work notebook (XP Pro SP3). I'm thus unsure if the equivalent Windows and/or IE Updates have been applied to both machines. Also, I don't believe that I have ever signed into my Google account from my work notebook, either.
 
The problem occurs after either (a) I have signed into my Google account, or (b) I have signed out and killed any Google cookies, but have reloaded http://maps.google.com/maps after having set a default location.
 
The first way I found "solve" the issue is to go into my Temporary Internet Files folder and delete "cookie:<my username>@google.com" and "cookie:<my username>@google.com/verify", then restart IE.
 
The info from the dialog box that is shown when I click on the yellow triangle/"!" icon down in the status bar is as follows:
 
     Line:     5
     Char:    372
     Error:    'elem' is null or not an object
     Code:   0
     URL:    http://maps.google.com/maps
 
However, I found that when I entered this URL into the address bar, it loads quickly and shows almost the same map that was visible when I set my default map:
 
http://maps.google.com/maps?ie=UTF8&ll=41.4373,-87.4831&spn=7.18561,46.953125&z=13
 
If enter that URL into the properties for my Google Maps favorite in IE, rather than simply http://maps.google.com/maps, then all starts to work well, maps load quickly and I can zoom, move around, etc. just as before even if I am logged in.
 
I conclude that something has been broken with the way the script takes info from the cookies for the default view when no lat/long info is present in the URL, perhaps?