i think you need a update for camera raw.
camera raw 5.X doesn't work for cs3, so you have to get 4.X.
download link for 4.4.1 which definitely supports the k20d:
Adobe - Adobe Camera Raw and DNG Converter : For Windows : Adobe DNG Converter and Camera Raw 4.4.1 update [edit]
found an update to 4.6. i believe this is the latest version for cs3.
windows download:
http://www.adobe.com/support/downloads/detail.jsp?ftpID=4036
mac download:
http://www.adobe.com/support/downloads/detail.jsp?ftpID=4033
greetz
shadeless