It should be www., generally, unless you're a huge site and then you can do whatever you want (cf. Twitter)

The reason I say this is for smaller operators who may not be running multicast'ed servers in 3+ geographical areas, then you'll probably want most of your traffic pointing to a CNAME of a CDN or at least a cloud-hosted load balancer.

I say "www." needs to die in a fire. It's ugly and pointless, and doesn't stop you from using other subdomains if you want them.

"www." is a good indicator in other media that a web address is being presented when it might otherwise be not entirely obvious.

Not entirely pointless.

I'm a fan of web.foo.com, personally. Much easier to pronounce. ;)

www allows for the use of CNAMEs (which, as I understand it, is why the www convention was established in the first place).

The convention of using relatively-short service-based host portions of an FQDN pre-dates the WWW. It was common to use mail, gopher, irc, whatever for various servers from a functional point of view.

Whether it was an A record or a CNAME record was generally at the adminstrators preference. CNAMEs had their own hiccups and weirdness to deal with, and still do to a lesser degree.