Now before buying a new bike from a dept store, or even a bikeshop, check out craigslist, I've seen allot of nice bikes on their that are hooked up, some of them only needing stuff like new tires, or a chain or just some tlc to get them going again for anywhere between 75-150...
As for walmart bikes.....
Frist off, the best of the 3 The Shawn White Thrash 3.5
I contacted the manufacture, and Confirmed that the hubs on this bike are sealed hubs, and headset, I did not receive any info about the bottom bracket though, chances are it might be sealed, if not, sealed bb kit and spindle is cheap enough. it has an integrated headset, and is drilled for gyro tabs.
next is the dk Dayton
Despite the picture, dk said when I called them that its a post production picture and that the bike has a standard headtube, unsealed fronthub, semi sealed rear hub, and a sealed bottom bracket.
the final bike is a Mongoose Mode 900
This bike does look like the best out of the 3 However, Mongoose confirms that all the bearings on this bike are unsealed which is kinda crap. despite it having double wall rims, and a half link chain, I personally think the bearings are more important.
and those are the top 3, other good entry level bikes are the mongoose 720, 540. and the Hyper SC1(if you can find it) DK Cleveland, Legend, and even the dk effect is decent.
(dk Cyngus from the dk website is also 279 at the moment. and I rate that above the thrash, a little bit goes a long way in dollars it looks like)
Bikes to stay FARRRR away from...Hyper Spinner, static, metro(static and metro are okay if your budget is that low) Mongoose Mode 90.100.180,270, and the LEAST worse of them all, The dk Bestwick, Dk effect,dk ratchet. and the Mongoose mode 360
"The D.O.S. System"
dk, Odyssey, Shadow.