I just bought a new iMac after no announcement at WWDC. My thought process is that nothing will be announced until after the "Major in Mac" program. I just do not see them adding another SKU to confuse the promotion. The poster talking about he/she would rather buy toilet paper is just being dramatic.
Bottom line is that you still get the same nice machine you based your puchase decision on originally. Its not like an update will render your machine useless. It will still work for many years, it will still retain resell value and it will still run leopard.
IMO, the update will be a form factor update and probably Santa Rosa, which is only good for an increase in RAM recognition (which isn't a big deal for my consumer needs...3gig is plenty) and increase in FSB speeds (not a big deal in terms of speed). They will probably update the video card, but I don't do anything now that even closely utilizes the power of the x1600 256mb, and even if they did, Apple tends to underwhelm in consumer video card offerings. In fact, I wouldn't be surprised if the 20" base models expected to replace the 17" had integrated graphics.
The thing that will have me kicking myself will be a price drop. If they bump specs, make the 20" the base model AND reduce the price, then I will come back to this post edit in how wrong I was and how much I wish I should have waited. However at the end of the day, I will go home to my 20" iMac w/chin and forget about whatever update and/or price drop occurred that day. That's how awesome I think the current iMac is right now.