A long time ago, I switched my Flickr account name to my Yahoo! screename. Why? Because they asked me to and because I basically use the same signin for everything anyway, so it really wasn't such a big deal.
And eventually, it will probably happen with MyBlogLog and del.icio.us, too. Seriously... who cares? It won't make me use any more Yahoo services.
And why don't I care? I don't care, because, at the end of the day, I'm glad the little startup where I store all my precious photos got bought be a big profitable company.
Yahoo! has lots and lots of servers and they're unlikely to ever completely go out of business. So, if I had a choice between signing in to Flickr with my Flickr vs. signing into Yahoo! and giving me a better shot I won't have to move any of those photos in the remaining 58+ years of my life, I'll take the Yahoo! ID and signin.
It's the same as when your bank gets bought by a bigger bank and they give you a new card with new numbers. It's a little bit of a hassle, but your money is probably safer at Citibank than it is at First Fifth Third Bank of Saskachawan.
I'm not pissed off. I'm thankful that such a great service exists, no matter what I need to enter in the box to login.