As a web-developer you don't have to worry about IE5.5 now, in 2008, seven years after the release of IE 6.0. Seven years to make sure your site works in IE 6.0. Seven years time to upgrade to IE 6.0.
If Microsoft goes through with what they've promised, one day you've got a web site which works on the latest version of IE, and the next day (when MS releases IE 8) you don't.
Since all web pages aren't going to be fixed in a day, practically nobody will upgrade.
The only solution is to do what they did for version 7, and 6, and 5.5. Maybe they could have prevented the problem by releasing a more standard-compliant IE 5, but since then, they're stuck.
The good thing is that we're slowly getting better at abstracting the problem away.
On the other hand, Microsoft's piecemeal approach to web standards is starting to hurt it. The de facto web development best practice is now "Develop for a standards-compliant browser first, then hack it until it looks right in IE." This hurts Microsoft because it means that most new webpages only work incidentally in IE; if a corner case of some new feature is accidentally missed in the IE rewrite, the workaround might well be: "Try using Firefox until we get a fix out."