... I promise to store them correctly.
Would you enter your Gmail (Yahoo!, MSN, etc) password in facebook for it to import your contacts or for any other reason? If we take a look at the numerous web sites that ask for our passwords of other services it looks like most of us do. I'm convinced that this is something that should not be done. There are others who think the same. When we begin to say that its OK to enter credentials in just about any login box you see, what we are doing is paving the road for more and more phishing attackers.
However things may be changing in the right direction. Yesterday when I created myself a Plaxo account and went to the sync settings here is what i saw
What you'd see just about everywhere.

and this:

There may be hope yet. Yahoo! is trying to make sure that if you're going to enter your yahoo credentials, its always going to be on a Yahoo! site! and nowhere else.