Internet advertising leader Advertising.com has lost several key employees recently. This is no inidication of the health of the company. Rather, it is a reflection on the industry. Of those that left, several have been with the company since it's inception. They have seen a hot market before and my guess is that while time isn't running out for new ventures, it felt like it. Advertising.com is not alone - any successful company, especially those that have been purchased, e.g. LowerMyBills are at risk and a prime target for recruiters.
Seasoned talent has a chance to still join successful startups and do even better than they did before. When Advertising.com was bought for north of $400 million in 2004, it seemed like an incredible windfall. They would go for double that today, only 18 months later.
Update 5/3/2006 - Scott and John Ferber, co-founders and entrepreners extraordinaire took a leave of absence from the company they created (and sold) effective 5/1/2006. I imagine it came as a shock to many, especially considering that many believe Scott could be the next head of AOL among other things.