Netscape first introduced the CENTER tag with version 1.1 of the Navigator browser (long before the DIV tag was invented). Because Navigator was the browser of choice for the vast majority of Web users, the CENTER tag was quickly adopted by others, and is now supported by virtually all browsers in common use on the Net. With the introduction of the DIV tag, the CENTER tag was supposed to go away, and in fact it's still considered "obsolescent" in the HTML specifications. In technical terms, <CENTER> is just an alias for <DIV ALIGN=CENTER>.
We don't expect CENTER to go away any time soon, so feel free to use it. It's convenient. The browser developers, especially Netscape developers, appear to have a lot more respect for the needs of the marketplace than they do for the needs of the people who write the specifications. Whether you feel that's a good thing or a bad thing (there are plenty of arguments for each of those opinions), the fact is that CENTER is here to stay.