While we sit back and wait anxiously for news of an 11g Mac OS X Client, or maybe even a full database, here is something to give us hope.
I downloaded the 11g Linux X86 Database. While reading the Oracle Database Administrator's Reference for 11g, I noticed there were several sections dedicated to Mac OS. This is a very generic document that has sections for AIX, Solaris, etc. I thought perhaps the document was a leftover from the 10g days because it has a section on Ultra Search, which I thought I was obsolete now, but is just deprecated in 11g.
Anyway, if you go down to section 2.2.1 "Automating Database Shutdown and Startup on Mac OS X", the script references 11g and a .../product/11.1.0/db_1 directory.
So at least the documentation folks are thinking about Mac OS X.