Well, I felt the same way as you and I DID wait until my son was 4. He had never been in daycare, and my husband wasn't working, so we didn't *need* to put him in preschool. I knew he was smart (though he didn't have much of a social outlet) and so I waited that extra year.
It is now one of my biggest regrets, though I thought I was doing what was best at the time. It's not the curriculum or anything that you have to worry about, but what you need to keep in mind (and what I didn't even consider) is that MOST kids these days are in daycare for a couple of years, THEN 2 years of preschool, even before they get to "real" school in kindergarten. By the time they get to kindergarten, they are familiar and well-established in these daily routines, and kids who haven't had the same experiences really struggle to fit in. My own son just started kindergarten and he's having a lot of problems; I get calls from school every day. Truth be told, if it goes on for more than a few months, I'd probably just pull him out and homeschool him. I don't know how I'd make it work, but I would find a way.
ANYWAY...:)
I totally understand where you're coming from; I cried my eyes out on my son's first day of preschool! Especially since he was like "Bye mom!" and just disappeared around the corner. :)
But if I'm to give you my honest opinion, I think you should send your son this year (when he's 3). You'll be doing him a favor, and it will be less of an adjustment when full-day kindergarten comes along...
I thought that I was doing the best thing by keeping my son with us for that extra time. And 20 years ago, that would have been true. But schools are really different now than when we were kids - they pack a lot more in much much earlier. Send him now, it's only a few hours a week, right? You're not overreacting, it will be emotional (for you), but you'll get over it in a few weeks, you'll see. ;)
Good luck.
(And after going back and reading others' posts, I realize I forgot to reiterate the best part - YOU CAN ALWAYS CHANGE YOUR MIND! If you decided it's too soon for whatever reason, you can always pull him out. It's not like it's mandatory).
Okay, good luck again!