I'll second Seb's comment, if you are setting up a dev environment for the next release of your product, you ought to be considering 2.30.
If you are worried about being on the bleeding edge, rather than the leading edge, a lot of the new stuff with 2.30 is additional to the product (Drag & Drop, Web Components, Silverlight, iPhone etc), rather than modifying existing code that occured with 2.10 (with its rewriting of the dialog statements) and 2.20 (the upgrade in the Qt version underneath the GDC).
To answer your original question, I'm not sure why it didn't get documented in the Migration Issues section of the documentation, but you'll find the answer in $FGLDIR/release/changes.txt, just search on libfgl.42x in that file