Ok, I'm a recovering geek (I went to college for IT but have a 'real' job now) but I just bought myself a new laptop and have helped several friends/family in the last couple of months.
A lot depends on your budget but here are some ideas for a teenager:
1) Sony Vaio (CR series)--great overall laptop, durable, last forever, come in cool colors (inc pink) for teens, about $1200
2) Mac--if you want a Mac, you can't beat them. But, they are not compatible with the Microsoft world so she won't be able to use to take stuff to/from school, etc. Overall, Mac's are great for graphics and photo editing, but unless she is studying to be a graphics designer the extra cost is likely not worth it.
3) Toshiba--can be a good laptop, they have a wide range of quality, from a $799 Ford to a $3000 Lexus. If you play with one and like the feel, chances are it will work great. The lesser Toshiba's tend to feel cheap (keys light, very weak hinge on lid, etc), if you get a mid-range or up they are reliable and a lot of people swear by them.
4) Dell--I'm going to have to agree with Darby and say, it is just not worth it. Sometimes they are great, but if something goes wrong, they are impossible to deal with. Dell has made a mint selling PC's to people without ever having to open a single store, but this means they also don't have to deal with customers face-to-face.
Ok, if you want more details or have any other questions, just ask or email me at tom_overstreet@hotmail.com.
Tom