OK, somehow the "real" compiler options are not [always] displayed properly in the compiler options window.
For example, if I start a new project in Lazarus, the Compiler Options (FI, FU) are right away set right. However the Compiler Options window is not showing any of it. When you click the "Show Options" button, you will see the FU and FI paths though. Jot them down (or copy them to a text editor).
Next thing I had to do was to close the new project, and open the converted Delphi project and manually add FU and FI and now the project compiles.
Is this to be considered a bug?
(Daily build 0.9.25 beta, Date 6/10/08, fpc 2.2.3, svn 15373, i386-darwin-carbon (alpha))