The main technical skill required of a Swarm user is the ability to write Objective C code. Other recommended skills include the ability to use tools such as gdb and emacs. For the record, if you know C and have some form of experience in either C++ or Smalltalk, then learning Objective C should take no more than a day or so.
Objective C references List of Objective C references on the Web. Includes a 10 minute overview to Objective C that is most of what you need to know about Objective C. Originally contributed by Nelson Minar
Object-Oriented Programming and the Objective-C Language An excellent (online!) book on Objective C. This is for generic NeXT Objective C: Swarm uses GNU Objective C in addition to our own defobj extensions.