Last time I left Makassar on March,2006 still no air rute Rantepao to Palu. Other option you can go by bus-named Fa.LITHA bus which is take almost 12 hours.
For some reliable travel agents in Makassar, I recomended Caraka Travelindo and Manorian Travel.
Caraka Travelindo
Jl. Samalona No. 12
Makassar 90174
Tel. +(62)411-318 877
Fax +(62)411-378 889
E-mail info

sulawesi-indonesia.com
Website:
http://www.sulawesi-indonesia.com
PT Manorian Sejahtera
Jln Cenderawasih 29
Tel. +(62)411-852169
Fax.+(62)411-856462
Hope it helps, and have a nice journey.
Salam,
Risma