Registration is not required to secure a copyright on a work. Copyright is secured automatically when the work is created. So the very fact that Mac OS X 10.5 Leopard exists is a copyright.
However, Apple must have registered their copyright on Mac OS X 10.5 Leopard before they can file suit for infringement on this copyright.
Read about all that at copyright.gov:
http://www.copyright.gov/circs/circ1.html
This would all seem to be moot though. According to copyright.gov, Apple registered their copyright on Mac OS X Leopard Version 10.5 on January 24, 2008, Registration Number: TX0006849489.
http://cocatalog.loc.gov/cgi-bin/Pwebrecon.cgi?v1=4&ti=1,4&Search_Arg=Mac%20OS%20X%20Leopard&Search_Code=TALL&CNT=25&PID=-H_wcyyigtEZ3UT-QtRpXsTJUefv&SEQ=20081222171112&SID=1
I have no doubt that there is something we're missing here, as it seems unlikely that Psystar's Lawyers couldn't do a simple search at copyright.gov for "Mac OS X Leopard". But this article offers little more than an announcement that Psystar has responded to Apple's copyright suit.
Always draw your curves, then plot your reading.