jQuery is undefined only in IE7 and IE6

jQuery is undefined only in IE7 and IE6

I'm running into a weird problem with jQuery (currently 1.4.3) and am turning to the community for help. When using Chrome, Firefox, and IE 8, jQuery loads fine for me (as it always has). jQuery loads after it, as do all our plugins. Page functionality is as expected. When I open open  Web Developer in IE 8, switch the Browser mode to IE 7, all heck breaks loose. Well, maybe not quite that bad but...

When the page reloads, I get a whole slew of 'jQuery is undefined' errors and the page stops loading. It also happens on a Winodws XP/IE 6 VM I use for testing. In both offending browsers I can view source, grab the jQuery URL from the source, paste it into the address bar, and download the intact jquery.min.js file. When I switch it back IE 8 mode, the page renders fine again.

To make matters worse I've tried switching back to jQuery 1.4.2 and 1.3.2, and I still have the same problem. 

My questions to the community are does jQuery have a debugging mode I can turn on to provide extra logging? And has anyone seen this crazy behavior before?

Thanks!