Pick yourself up a used Thinkpad T20 from ebay.
In most cases, I'd never condone a used notebook purchase, but in the case of thinkpads, I think it's safe. Thinkpads are VERY durable and last forever.
Definatly email the seller asking about screen condition. I bought mine, and it looks like they used an alcohol based cleaner (Windex) to clean the LCD screen. That causes some yellowing of the lcd, so colors are a bit off. I don't notice it with DVDs , but on a pure white background it's fairly evident.
The T20 is a p3 700 with 128mb ram standard and a DVDRom. I get over two hours of battery life on mine (which is almost unheard of with a used laptop battery). That's with my 802.11b card in it and using the internet. It's thin, light, and looks pretty cool . Paid $600 shipped for mine, it has 384mb of ram.
It's great for DVDs/office apps/net surfing/etc.
If you want to replace your desktop, go with a newer model, but to suppliment an already good desktop system, I chose a less then top of the line notebook.