No doubt about it, popularity of popup blockers is rising above the ignorable level for those who use it for legitimate reasons.  Google Toolbar and Mozilla/Firebird are two primary causes.  Content-rich services like AOL, MSN, and Yahoo are likely to add fuel to the fire with controlled popup blocking which will block everyone else's but their own and their partner's.

While I hate annoying popup ads like everyone else, I feel that blocking all popups amounts to throwing the baby out with the bathwater.  Popup is a very useful tool in UI designer's toolbox and it would be a shame to lose it or resort to using complicated pseudo-popups, artifacts that just looks like a popup window.

For now, I am advising everyone to avoid using popups until we can find a cheap solution.  One solution I am looking at now is the use of copyright law to discourage use of popups for advertising by businesses.  The idea is simple:

Copyright and restrict use of a unique string or image that popup blockers can use to recognize legitimate popups.

The ultimate cause of popup ads is money.  And, where there is money, there is usually someone who can be sued.  While I hate unnecessary litigations, I prefer simple social solutions to complicated technical solutions.  There is a major flaw in this solution though.  There is no powerful industry association like RIAA to stab the legal jeopardy straight into the heart of popup advertisers.