I started the AT NOBO in late Feb last year (2010), and most or all of the advice I've seen here thus far really seems right on to me.
Well, I could quibble with the tent. I started with a Gatewood Cape as both backup shelter (I did indeed sleep in shelters almost the entire trail) and as rain gear, but as the south was hammered pretty hard by more snow than usual in 2010, there were lots of blowdowns, so I swapped for my Contrail (having your only rain gear and shelter shredded by blowdowns isn't a great survival strategy). And it turned out that I virtually never used the contrail, very seldom at any rate. My hiking partner Lucky was ultimately smarter with his SMD Wild Oasis --- a lighter backup shelter to carry and very rarely use. I guess bottom line is that the more confident you are that you can always make it to a shelter at night, the more you can balance risk/reward in carrying a lighter (not four season) tent or tarp. YMMV, don't haunt me if you get killed this way ... ;-)
Seriously, a well pitched tarp could be a great choice for a person with the right gear to mostly sleep warm in shelters.
I used a MB Ex Light down jacket for the entire CDT this year, again in a pretty tough winter year, and I was fine with that. On the AT with an early Feb start, however, I too would want something warmer; I think there were few if any nights on the CDT this year as cold as I was in late Feb and early March at times on the AT. But I also used a WM 20F bag on both trails, so having a warmer down parka on the AT was not just for general "in camp" use, but also a truly essential part of my sleep system. As were my down booties on the AT at the start (didn't use them on the CDT this year).
I made due for padding with a regular size Neo-air plus a couple of thinlight pads, and this combo worked for me down into the upper teens, but likely not so well much lower. It was nice, however, to have a multi-piece system here where I could mail home thinlite pads as it warmed up; the same approach worked great on the CDT this year too, starting with two thinlites and ending up eventually with none.
One thing about a warmer parka is that mine (MB Alpine lite) was just flat too warm to ever wear hiking, whereas my Ex Lite this year was something I did wear hiking on occasion. On the AT I used a combination of MB thermawrap vest plus windshirt to walk in when cold, removing the vest if/when I warmed up enough, and the vest of course could layer under the parka (not a huge gain in warmth there over the parka, but at low temps you take what you can get!). In terms of clothing to hike in, btw, make sure you're prepared not only for cold temps but for cold wind. Think ear protection (I personally like earbags to augment a couple of hat alternatives), and *mittens* --- not gloves. I'm a big fan of Dachstein mittens now, with some sort of light (I used eVent) shell mittens.
I liked using breathable trail runners throughout on all trails, using at times goretex or VB socks, but mostly just wool socks. Differing opinions on this, some very experienced folks prefer a boot when it's very cold out. I personally like wearing shoes that work well for my feet at pretty much all times ("boots: just say no").
I echo the sentiments of whoever recommended Kahtoola microspikes. I'm not sure if the topic of snowshoes came up here, but with an early Feb start it seems to me that it's possible you could find yourself needing those in places, or perhaps having to do a roadwalk or blue blaze route to get around deep snow or perhaps to flip. Tough to know ahead of time. Early starters in 2010 mostly or all did a (also partially snowy) roadwalk from Clingman's Dome to Newfound Gap due to butt-deep snow there that no one had busted through yet, but that was the only time I would have wanted snowshoes with a late Feb start that year.
It's pretty cool being out ahead of most of the pack; the folks that were out at that time were pretty cool too; i.e., one of the benefits was that apart from easy shelter space and just generally fewer people out there, I really enjoyed the company of the folks that were out; I think there was a bit of a special bond there.
Best of luck, I hope it's a wonderful trip for you!