I would not recommend a netbook because it is too small and not powerful enough. If you want portability buy a 12 - 14" and if you want to do more than wireless cracking buy one 15 - 17" with a resolution of minimum 1600x900.
I recommend a good resolution, because you'll want to have more windows open in the same time (example: 2 shells and a browser, or a browser and burp...).
Also, don't put too much in a laptop unless you are rich.
For example, I just bought an used laptop, Dell Precision, core 2 duo, 4GB RAM, 17", SSD and I will use it for normal browsing and I have 2 OS windows and Backtrack.
Next week I will buy a server, dual CPU XEON, 6GB RAM, 2HDD and I will put my VMs, I will use it also as an web server, as a test machine...
The total price - 650$
If you buy a good laptop 17", it will cost you more than 1000$, without any return on investment. Worst, in one year it's value will be 700$ max.
I am giving you these advices because I saw that you are on a budget. Otherwise... buy the most expensive MAC, put a Linux as a dual boot and .... you'll do the same work as I do
PS I am wondering if in few years I will have the same judgement