A port of SDL to AmigaOS is now available in the latest CVS snapshot:
http://www.libsdl.org/cvs.html
This port was done by Gabriele Greco <gabriele.greco at galactica.it>,
and is not yet completely finished. The CVS snapshot contains code,
but no build infrastructure for the Amiga.
I’ll make a more official announcement when the Amiga support is a
little more mature. 
Congratulations to Gabriele!
See ya!
-Sam Lantinga, Lead Programmer, Loki Entertainment Software