For on the go sketching I use a Samsung Galaxy Note 10.1.(around $500). It was a wacom digitizer built in, and it comes with the S-pen(stylus). You can even get an S pen that was designed by wacom/samsung. I use PS Touch and Layerpaint but there are many more pressure sensitive painting programs. It does't come close to being a replacement for my desktop but it's good for creating a starting point for an illustation. If you have a large budget there are a bunch of tablets that run windows (ASUS Eee Slate is one) so you can install anything that you would be able to on a desktop. They have more ram and computing power. Those run about $1000 to $1600.
There are videos on youtube that demo each one of these options.
Just a warning. Android and IPad tablets are not really laptop replacements. It depends on what you are using your laptop for. The Windows tablets are basically the same as a laptop. I don't know much about the new batch of Windows 8 touch screen laptops or Windows surface tablets but you might want to look into those also.