Here is an interesting story about the current Flickr and Yahoo! situation:Flick Off!. Basic outline: Yahoo! bought Flickr in March this year and now announced, that all Flickr users will have to register with Yahoo! starting 2006, even if they had been with Flickr for a while.
A heated discussion of as many as 910 of the 1.3million flickr users has started. About Yahoo! integrating their bought services (not allowed!) while Google also does this (err, not so bad…).

Really, if you read the official flickr blog, nothing much seems to change. You need to sign up with Yahoo! and you need that login in future for flickr. But you’re screen name doesn’t change, the copyright of your photos still belong to you (unlike when Yahoo! bought geocities and all of a sudden claimed copyright for all the geocities content), and you also keep your flickr URL.

So why do people bother so much? Why does run across the blogosphere?
I guess it’s because people are so involved. It’s not just a brand or service they buy and use. It’s their creation. So far, they could influence flickr by adding photos, posting ideas for new features, shape the whole community. That way, flickr seems to be everybody’s creation, every body is participating in shaping flickr. So it does indeed become really personal:

Millions of people now interact with one another via computers on networks, where they have the opportunity to talk, to exchange ideas and feelings, and to assume personae of their own creation.

(A quote from here )

Fact is, the way Yahoo! starts to behave now is much like it would be in a badly communicated corporate takeover (which it wasn’t), when the employees start wondering what will happen to the future of their company (I have experienced that feeling myself!).
The way the users have helped shaping the community, it is „their“ company. And now they wonder, what Yahoo! is doing, what else they might have planned. The feeling of a loss of control over their own creation and hence „personae“, even if this community is just a little part of their digital identity.

So in effect I think it is much more of the great „what else is coming?“-scare, rather than the fact that people need new login IDs.

[added, a few minutes later:]
Imagine this situation in the non-communicative offline world of brands. One softdrink brand buys another. They change the logo, the distribution channels, whatever. The target audience (at least those that feel strongly connected to the brand) can only express their disgust over the merger by writing to the company, tell their friends (on the phone or at a social event) or tell market researchers (if their lucky enough to be picked for interviews). So, how many mergers have happened in the past, that have not had that much controversial discussion?
It’s the other phenomenon of digital culture: companies, that have their product within this space need to be much more diligent in terms of PR and comunication. Because thousands of negative opinions are just one click away…