Nearly any name brand notebook from $900 to $2000 should have wi-fi built in now. Make sure you get a notebook that has 54G/b -- don't get one with the old 11b standard. The card is often identified as the Intel 2200b/g or equivalent.
Check out Circuit City, Best buy, etc. When they have sales they have some good options.
I have the Compaq X1000 series which is equivalent to the HPZT3000 series (Centrino Pentium M notebook). If you order custom online with the $100 custom order rebate you can get them in the $1200 range (and they come with up to 64mb dedicated video (don't get integrated or shared video if you can avoid it).
HP/Compaq also sells an Athlon 64 notebook which is a good alternative also (cooler and lighter than Pentium 4M notebooks but not as light as a Pentium M centrino).
HP/Compaq has an additional 6% student APP rebate for college students but find out if that applies to high school or middle school as well. See hpshopping.com and X1000forums.com. You can custom order the notebook exactly as you want it and not waste $ at hpshopping.com (or you can custom order Dell or Toshiba notebooks as well).
Whatever you choose, if you want to have the most flexibility with the notebook make sure you get at least 64mb DEDICATED video RAM. You can't add a video card later like in a notebook. Cheaper models have integrated (shared) video but this will limit you.
You can still get the notebook above in the $1100-$1300 range with 64mb Dedicated video -- especially if you order only 256 mb RAM and add a 512mb PC2700 RAM stick later yourself for under $100 to go to 768mb RAM total for less.
I need an inexpensive notebook for a middle school student. I think I want it with built in wireless. Can anyone recommend a low-cost solution?