I did a quick test (few weeks old CVS). I tested mainly chat, npc chat, system messages from greeting, fighting, advising. No tells/guild/group. In general it seems to work as I imagine the idea is. Text appears in the main and original tab for the selected choices. Tabs flash only when selected to do so. The only thing I noticed that is not correct is the following:
If you target an NPC and type text in the main tab, the NPC does not hear you (as I would expect). Typing in the NPC type makes the NPC hear you. However, if you direct NPC text to the main tab, this tab is activated when the NPC answers you. If you then want to talk back, you have to first select the NPC tab again. I do not think this is intended behaviour.
I understand this is an 'easy' client side fix, so people can direct everything to one tab, and stop the flashing. I think it would be even nicer if the flashing was a bit smarter though. I would like a tab to flash if it shows me there is new information there, that I did not see yet in the active tab, and only flash in one of the tabs. I cannot do that yet with the new flash options. For example, if I redirect system messages to main, and I am in main, I do not want to see system tab flashing when someone greets. However, when I am in the tell tab, and someone greets, I would want the system tab to flash (but not the main also as in the current official client). Just an extra wish to be ignored

Good job on getting the tabs somewhat configurable anyway
