Indeed, still a good 15% of Mac users on that chart are on 10.5.x - too high to just drop support. When it gets under 5% that'll be the time.
Cas

According to those stats:
- Leopard was
14.39% of all OS X installs back in March 2012.
- Last month, July 2012 it was down to only
11.37%.
If you look at the Leopard market share
trend for the last year from the same stats, you'll notice Leopards market share is dropping at a rapid and constant rate (was
21.21% back in August 2011). Its likely this trend will continue and its market share will be <5% in just a few months.
Another source
here has Leopards last months market share at
10.4% and also confirms the constant and steep loss of market share
here.
Also another thing to consider, princec mentioned earlier in this thread that Steam requires OS X 10.5.8+ as a minimum, so out of those OS X 10.5 installs above if we look at the version break down
here, only 71% of those Leopard installs are on 10.5.8+ (considering that update was released 3 years ago, its unlikely the current user base will be updating), so we can knock of another 1/3 of the above market share bringing the roughly
10% market share down to about
7%. You could go further on that 7% figure and consider how many of those Mac users will actually be using a 5 year old Mac OS for gaming, I'm guessing it should put it below 5% (matching
this stat)

.
Further news stories like
this (1 in 10 macs already on latest OS X 10.8, released just 37 days ago ) show mac users upgrade pretty quickly (compared to windows) especially due to the cheap price of the upgrade.