Not one single 3rd-party has announced a Leopard-only product. Since when did Apple announce that they were dropping support or security updates for Tiger?
This is just baseless banter!
Are you kidding? Even my stupid trivial simple applications with garbage collection break Tiger support. That being said, yes, some 3rd party devs have announced Leopard only versions.
The biggest one (for me) is TextMate, which is an app I constantly use:
http://macromates.com/blog/2006/20-will-require-leopard/
The second one is Delicious Monster:
http://arstechnica.com/journals/apple.ars/2006/12/13/6275
Here's a third, FlySketch:
http://www.gusmueller.com/blog/archives/2007/01/2007.html
Name anything else using the new GC in ObjC 2.0 or Core Animation and it will not work unless the dev intentionally works on a Tiger-compatible version. And just wait until Leopard's been available for a few months. Panther support dropped in lots of apps, maybe even Tiger. Just like how some apps now don't work on PPC, you get the idea.
Also, see
http://docs.info.apple.com/article.html?artnum=61798 - Older versions of OS X eventually get dropped. Sometimes it's because whatever software needed a security update isn't present, and sometimes it's because the software is too old. Unfortunately Apple doesn't like to publicly announce product end-of-life dates, but it is inevitable that development+bugfixes stop coming, and that eventually the security updates will end too.
Microsoft, on the other hand, has no problem disclosing that information. Windows XP Professional security updates will be available to all customers until April 8, 2014 in the United States (
link). In comparison to the last security update I see for Jaguar...
Point being, I don't want to run software that's not going to be updated in any way except security-wise minimally for a short time, compounded by the fact that Leopard has a LOT of new security features like sandboxing, ASLR, code signing and NX stack/heap/etc.
If you've ever had to admin a network filled with outdated installs of software, you'd know why I so fervently hate outdated installs.