While I don't think you should support IE6, since market share for it is really really low, I think it's still important to support IE7, 8, 9 and 10. If you don't, you're just gonna make people mad. And since phpBB isn't just developing for the "developers community" (who usually run lastest browsers), they need to keep in mind a lot of forum users that use phpBB don't even know what the word browser means.
Now, IE7 more and more is less important, but I still support it to this day the best I can, even though I hate it's guts. IE8 is a must because of the large amount of Windows XP users, remember they can't update to IE9/10, so it's either IE8, Firefox or Chrome. For Chrome, I don't bother supporting anything more then the latest since it updates itself, and the last few versions haven't seen much changes as far as standards. Chrome has supported CSS3 for a long time. Firefox I test in 4.
That doesn't mean I don't use CSS3 shadows and stuff even though some browsers (like IE7 etc), but shadows is not needed to make the style usable, and to not have them show up in some browsers shouldn't make the style unusable. if it does, you should look into making a new style because those type of elemets should be more eye candy, not make the style. So same with rounded corners. Even with older browsers such as IE7, they wont get the rounded corners, but that doesn't matter, since it's really not a requirement to make the site usable.
Formerly known as jsbean.