How can I write in a PDF with a tablet and a pen (a.k.a  Is there a Xournal alternative for Android)? There is already a similar question, which is about the Xoom and note taking. But I'd like to ask this in a more general way: 

Given a tablet (and a pen), how can I write - not just adding notes and comments, more like one would write on paper - in a PDF?

The additional writing should be saved within the same PDF file and readable/showable by the most common PDF readers. Essentially I am looking for an Xournal alternative for Android.
So far the closest that I found to Xournal in Android is RepliGo Reader or ezPDF Reader.
They both


Allow drawing of lines, curves, freehand, etc.
Save the annotations/drawings within the PDF file


But in my opinion both Apps do not use the full potential of what's possible with the Thinkpad Tablet and/or xournal. It may be subjective, but my colleguas who have Thinkpad tablet PC's running Linux with Xournal seem to have a better experience "writing into PDFs". I hope that there is more to come on Android.

Alternatives, suggestions are always welcome. :-)

Edit: I am going with ezPDF Reader. It's the best PDF reader for Android IMHO.