The imagery that comes with saying "OS X" is too prominent in todays tech world. "OS Eleven" comes off as sounding less interesting, to me at least.
Why, I worked with System 6! (Autographs upon request.)
I also wanted to mention that this question is rather ridiculous. Nobody here knows whether the next version will be 10.6 or 11. Nobody knows what will happen with Apple until it happens..
I also saw that Steve Jobs said there might not be another OS update for another 10 years or so.
If that is so, i think 11 would be more realistic then 10.6