As for CSS3, you can use certain features and construct the site so that it degrades nicely in older or less capable browsers. For example, if you use the border-radius property, the site will work in less capable browsers—they just won't show your rounded corners.
If you're building a web app and have a lot of time (and preferably get paid for it) you can always use the features of newer browsers when available, and if they're not available you can use alternative methods (e.g. images for rounded corners). That way, the site works/looks pretty much the same way in most browsers, while the newer browsers take advantage of their more modern features.
I agree with ralph.m when he says that those who learn it may be "ahead of the game" later on. If you've more or less mastered current standards, why not continue learning something new?