How can I write in a PDF with a tablet and a pen (a.k.a  Is there a 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.