Upsert (update or insert) in Sybase ASE?

I'm writing an application to move data from Oracle to Sybase and need to perform update / insert operations. In Oracle, I'd use MERGE INTO, but it doesn't seem to be available in Sybase (not in ASE, anyway). I know this can be done with multiple statements, but for a couple of reasons, I'm really trying to get this into a single statement.

Any suggestions?