Spammers encoding e-mail addresses in URLs?

Here's a spam-related issue I've noticed and been wondering about since reading a posting by pinniped.

I love KDE's KMail mail client. It not only does a better job with multiple mail accounts and identities than Thunderbird (don't mean to start a flame war, just IMHO), but it also allows me to forbid any e-mail from accessing the Internet to load image files or other 'net-hosted content.

Since I have KMail set to just display plain text mail (I don't mind the extra click to view the few non-spam HTML mails I get), I get to see the URLs in e-mails that make it past my spam filters.

Many of those URLs have long hexadecimal (I assume) strings in the path/subdirectory portion of URLs and even in the paths of image files.

My initial thought is this: those hex portion of URLs are being used to ID my individual e-mail address. Hell, I'm rusty, outdated and just suck at coding but I know I could write a script to do something like that.

My thought is that spammers are encoding my e-mail address, and have a server-side program to reference that against a database to get "feedback" on what spam I'm reading. If that's true, I don't have to click on anything for them to get their feedback -- all I have to do is to view/read an HTML e-mail with an embedded URL to an external, 'net-hosted image.

Questions:

Are spammers going to this much trouble to do such things?

And if so, how widespread is the practice?