I consider such breaking up of the CSS to be LESS useful as you're hunting all over the place for properties of single items / item types... you took the color and you threw it over there, and you took the size and you threw it over there and you took the typeography and you threw it over there... as if that's magically somehow easier to maintain?
Tin man: Oh, that's you all over
(No wonder some people rely on firebug for their own code).
Multiple files just makes it worse for one simple reason -- HANDSHAKING. the more files you have to request to build a page, the longer it's going to take to load REGARDLESS of the size of those files or the bandwidth of the connections at either end! EVERY time you request a separate file the browser has to request the file, ack the request and say send it, then start receiving the data -- each of those can take basically the 'ping time' to the server so you're talking anywhere from 200ms to 3 seconds 'overhead' per separate file. While browsers can request multiple files at once (typically 4 to 8), you get on a work or public connection where it's already bumping up against the connection limit (or if you're just running a torrent client in the background) that ability to do multiple requests at once gets GREATLY reduced.
Think of it like with FTP, which typically has much MUCH more overhead than HTTP (though it can be less if you use LOTS of cookies) but works much the same -- ever notice it takes longer to upload thirty 1k files than it does one 30k file?
As a rule of thumb I consider it bad practice to have more than TWO stylesheets per MEDIA target. You are coding for MEDIA targets, right?