You, rkaufmann87, might try taking a little gentler approach, i.e. you don't have to insult people. Obviously, if you are right, that would be evident from your useful replies, so no need for the snide attitude.
The problem is both of you sent me to the same page on Apple, where nothing is said about 27-inches or "late" 2009. In fact, it specifically includes 24-inch iMacs from "2009," not "late 2009."
And I only reposted my question in order to correct the error that you (correctly) pointed out (my use of G5). No slight intended to you, and I'm not fishing for a response that I "want to hear," since that would obviously be useless.
I am finding other websites that agree with you. If this is true, why doesn't Apple clarify this in their page about target display mode? I have suggested they do so just now.
I have a call into Apple help via phone tomorrow. I'll update if they corroborate your statements. You could be right, but it's very annoying when several Apple chat agents tell me it's possible, that I have the wrong cable, and yada yada.