10.5 will not be the last version of OS X unless there are no updates for about 13 years. since Steve Jobs has said that OS X will be the Mac OS for 20 years and it is just geting up to 7 years old.
Well, thats is you ignore the fact Apple worked on OS X before it came out, and will support it when its no longer sold....So I'd save we have much less time then 13 years