But they would probably see abc.foo.com as different from xyz.foo.com

The www. prefix is a special case for people; it literally means "its the world wide web" or "its the internet" to them. Technically, the subdomain is irrelevant. But, most people don't know this. They don't know how the internet works and (I'd suspect that this is true in more cases than you might think), think that www. is needed to get to whatever site they want to visit.

Of course, this is all assuming that they even type the address in, rather then searching.

I generally just type in the name of the site that I want:

'amazon' for www.amazon.com

Firefox is generally smart enough to take me to where I want to go. The only issue is when some hack site games it's way to the top of a search result for something else, but I've only run into that a few times.

Which is itself an issue -- since many browsers are now turning the 'address bar' into a combo search+address bar, novice users may not even realize what the difference is (or was).

You are right - but I think there is a difference in that omitting http:// doesn't break so many things.

Try this link: http://www.news.ycombinator.com for example - this is what happens to most addresses not prefixed with www. This means that hiding the www. would mean waiting to work out that the site doesn't exist (or the connection is down) before trying the non www. version, and worse that you would get the wrong site if www.foo.com and foo.com were distinct. Hiding the http:// doesn't have such massive implementation issues.

Right. My point was supposed to be that the average user doesn't know about these implications. And since it actually makes a difference in what page is viewed, don't hide the www. from them. Sorry if I wasn't too clear.

What's worse is that a lot of sites will dns-map foo.com to www.foo.com properly, but the ssl certificates will only be for www.foo.com.