Q: How will security on older iMacs that are not eligible for Mountain Lion be affected?

Eventually Apple will stop issuing security updates for OS X Lion -  all developers of operating systems eventually stop, though Apple's OS development schedule tends to be more accelerated than most -  but Apple probably will continue to issue updates for Lion for quite some time to come. They still are doing so for Snow Leopard, and even for Leopard for truly critical issues (such as the recent Flashback trojan).

If you're still on 10.7.2, by the way, then you're already behind on several important security updates.

Regards.

Message was edited by: varjak paw

Not at all in terms of your original question. I only mentioned it since you seemed concerned about security updates, but your profile indicated that you were still on 10.7.2.

Regards.

Recent past behavior has been Apple only provides security updates for the current and the previous OS versions (right now 10.7 and 10.6).  As varjak paw mentions, there was a minor exception to this with the recent malware threat, although even there the security update was only for Intel models and not PPC models running 10.5.

Since Apple has brought out a system upgrade roughly every two years for the past decade, this has meant the last OS version that a computer would support gave that computer a roughly 2 versions x 2 years = 4 year retirement period as far as support went.  Since your computer cannot be upgraded to Mountain Lion, its retirement period has started.  However, now Apple is releasing ML within a year of releasing Lion. Will the 'last two versions' policy still apply for updates, effectively giving your computer only a two year retirement period?  Will they change that too?  We don't know.

Vintage and obsolete products - http://support.apple.com/kb/HT1752