MS says that XP SP2 was changed to turn off clientcaps version checking for flash:
http://www.microsoft.com/windowsxp/expertzone/chats/transcripts/04august12.mspx

But perhaps this means that XP SP1, Win2000 (have not checked this one myself), Win98 would work.

So then the only hole would be Flash 6 on XP SP2 (and all Later versions of Windows to come). But the number of SP2 systems and future Win systems with Flash 6 must be even smaller.

If this turns into a bigger issue (I really doubt it will) then maybe I’ll adjust a bit, but for now I haven’t received more than a couple complaints about it.

ClientCaps cannot be used on WinXP SP2 to obtain the flash version, but it appears to be possible for previous Windows versions (I tried it on IE 5 Win98SE).

Here is a sample tester using the clientcaps when version 6 is present (but uses GetVariable for other versions):
http://www.pinlady.net/stuff/FlashTester.htm

MS says that XP SP2 was changed to turn off clientcaps version checking for flash:
http://www.microsoft.com/windowsxp/expertzone/chats/transcripts/04august12.mspx

But perhaps this means that XP SP1, Win2000 (have not checked this one myself), Win98 would work.

So then the only hole would be Flash 6 on XP SP2 (and all Later versions of Windows to come). But the number of SP2 systems and future Win systems with Flash 6 must be even smaller.

Don’t know how much of a difference this makes though. Most people are probably using SP2 anyway.
But I doubt anyone would install Flash 6 on Vista.