Freedom to Tinker mentions a company called readnotify.com that also uses web bugs, but also uses IFRAMEs, which cannot be disabled by just turning off remote image loading. There's also a mention of how these places can put tracking bugs in Word Documents.

readnotify.com is an email tracking system that uses Web bugs (like didtheyreadit) and also uses a trick involving IFRAMEs (unlike didtheyreadit). The IFRAME trick cannot be disabled by the standard countermeasure of turning off remote image loading. There may not be an easy way to disable it in today's email software, short of turning off HTML email entirely.

Worse yet, readnotify offers a service that lets anyone put hidden tracking bugs in Word documents, Excel spreadsheets, and other OLE-compliant document formats. When somebody opens a document containing one of these trackers, the time of the access is reported, along with the accessor's IP address (which often reveals their geographic location) and some configuration information about their computer. [Freedom to Tinker]

So my question is, is there an easy way to turn this stuff off in Word/Excel? And I guess I'll have to see if Mail.app supports IFRAMEs.