SeekSift uses a reverse captch/honeypot (explained here: http://www.nedbatchelder.com/text/stopbots.html) and it works really well, w/o the annoying user interface problem you've described.
Interesting, and good, ideas. I think they could be defeated by a well designed spam bot, but most (if not all) currently used techniques could as well.
I haven't used a screen reader, nor have I looked at their source code, but it did make me wonder if such a setup (with user invisible fields) might still be read by the screen readers (depending on how they convert the raw html into content for the listener).
If I ever get around to fixing my comment system on my blog (I don't really like comments... and thats probably why my current commenting system is passive-aggressive), I'll try implementing these ideas. Good link.
