John Gruber of Daring Fireball has a great discussion (as always) on Mac OS X major version use:
The 10.4 usage (36.4%) for Omni seems very high and developers might factor this into their decision to continue supporting 10.4. However it can be argued that Omni stats are skewed due to the Apple bundling agreements that existed while 10.4 was being shipped (these agreements ended before or early in the 10.5 era). All Mac hardware shipped with OmniGraffle during 10.4 (but not during 10.5) so the overall market share of 10.4 is probably lower.
If you're a developer (and according to Gruber 11.2% of his readers are) you might not be fortunate enough to be calling your own shots and have some 'platform agnostic' marketing guy or product manager telling you that you must support 10.4 because it's a significant portion of the market or your partners need it.
Supporting 10.4 is probably a bad move for most developers at this point (unless developing for education - maybe). Here's how to argue the case for 10.5+ against someone not fluent in the Mac platform.
1. Your software won't be as good if you're developing for 10.4
The economic problem is always the same - limited resources and unlimited needs. Whilst you can work around 10.4 limitations, doing so means you've got less resources allocated to other things - like solving your business problem better. There's some really cool tech in 10.5 - from the big stuff like Core Animation and NSOperation to the small stuff like gradients and bezier paths with rounded corners. Your software will almost always be better from using these newer technologies.
It's not just the development effort either, you need to QA and support 10.4 too. That really sucks for the poor schmucks who have that job (probably one of whom is you).
Make the choice: Do you want to make good software for 10.4+ users or great software for 10.5+ users? If you're still thinking you can do both, then re-read the above until you realise by definition you can't. Your software will always be better if you develop for 10.5+ because you'll have more resources to spend on solving your business problem better or making your software easier to use.
For many products, you'd get more users from better software than you would from legacy OS support, but that argument really hinges on how many users (particularly paid ones) you're likely to get from supporting an older OS, so market share stats might be important.
2. Current market share
There's no definitive source of how much of the overall market is 10.4 or 10.5+. Omni's stat (36.4%) are probably skewed towards 10.4 due to the old Apple OEM deal and Adium's stats (15.3%) are probably skewed towards an 'early adopter' crowd who stick with the latest OS. So let's assume the overall 10.4 market share is between 20% and 30%.
3. Future market share
By the generous Omni stats, 10.4 is showing a roughly linear decline from 50% on 1 Jan 2009, translating to a decline of 18% per annum. In a year from now, 10.4 will be really inconsequential. You'll need to make back all of your extra costs from these users the next year*, or you're at a loss. This isn't just the explicit costs like development, QA & support, it's the implicit costs too - like your software not being as good. Add to that the future costs of updating to newer technologies in 10.6 like Grand Central that are automatic if you use NSOperation (which is included in 10.5).
*If you're at the stage of deciding whether to support 10.4 or not, then it's probably a few months before your software ships anyway so you need to take into account what the market will be when you launch the software.
4. The OS market share argument is rubbish
Just like the Windows vs Mac market share rhetoric, the total market % isn't important anyway. What's important is the market share of users in your target / addressable market.
Think about it… Any consumer** using 10.4 is probably too frugal to buy Apple's software, a late adopter or using an old system. Do you really think they are going to install and (more importantly) buy your software?
**You'll notice the emphasis is on consumers. There are still some big educational customers with 10.4. If this is the case for you, then you should probably take the decline in market share into account and make an assessment based on that. You might still be stuck with 10.4 for a while longer, but make an informed choice.
But what if I already support 10.4?
This stuff is all still relevant. If you're keeping your software up to date, then you'll probably realise a lot of this already.
As Wayne Gretzky said "skate to where the puck is going to be", this is still relevant to all of your current and future development efforts.
There are some 10.4 users and some of them might buy your software, but are there enough to regain the costs, time and headaches of developing for 10.4 worthwhile?