So, here's my story:
There was once I installed Ubuntu 8.10 on an Acer tablet laptop with a wacom digital pen. In order to make the digital pen work (fully functional, e.g. with the ability to detect pen tip force), I installed a driver provided by the Linux Wacom Project (http://sourceforge.net/projects/linuxwacom/). It seemed to work very well. Although one can use it as a mouse, it provided a much higher resolution. In addition, it was pressure sensitive. It can detect the force applied on the pen tip. This can be verified by using a painting software such as Gimp. One can use a brush and see the stroke width variations after enabling the pressure-sensitivity option in the configuration.
The same driver works for tablets as well. I have a wacom tablet pad which uses a USB cable to connect my computer running Ubuntu. I have been using it to collect digital ink data. It worked fine.
If you are looking for a digital pen with which you can write on real papers, you may want to take a look at Digimemo (http://www.digimemo.com/). I had seen it worked on Linux by using some open source third party software. The software doesn't come with the product, but only for offline use. You can load the data into Journal and some tools. The format is very simple, so it is easy to write your own transformer code.
I hope this will help you.