Actually, i do not think that you can install Linux on a ibook or any Mac for that matter.
If you are planning on dual booting with linux, then you will have to go with the Dell. And since the dell will come with windows XP, that will give you a good platform for different programming languages and if you dual boot with Linux, that just gives you more posiblities.
As for the ibook, i have not heard of any linux distro that will install on mac hardware. Plus, your programming language choices may be very limited on the mac platform (that is just my guess though, i could be very wrong).
So go with the Dell or other PC based laptop.