cminblues - 2:01 pm on Mar 23, 2004 (gmt 0)
I think they _are_ a tracking device.
Say you send 1 million of random-words spam emails.
The prog you use for sending that, keeps track of the semantic areas of words in [the subject of] every email sent.
And, because [I suppose..] each of these emails has at least an external image inside ['img src=http://etcetc/uniqueid'], then you're able to summarize which semantic areas are more 'fruitful', in terms of people who open these emails.