This is a feature in vbulletin and I find it quite useful in identifying search engine spiders.
Possible to add this feature in IPB 3.1 ?

There is a modification for something like that, have a look at this:

(T30) Display Members Browser 1.0.0

That modification will only show included user agents, It doesn't solve the purpose.
I want to identify and include 'user agents' that aren't already included for skin mapping and few other things.

I'm not 100% sure what you're actually asking for, and probably won't be alone. :)

Can you provide a link to the feature ? (I'm assuming it was an add-on / mod for VB)

Here is a screenshot of vbulletin's Online member/guests List.

This is pretty easy to achieve with a very small skin edit, since this info is already made available to the skin template that displays this list.

http://forums.codersrefuge.com/tutorials/article/41-show-user-agent-in-online-list/

Great ! Thank you Micheal, You have helped for the second time today.

Great ! Thank you Micheal, You have helped for the second time [url="http://community.invisionpower.com/topic/303218-cuiltwiceler-user-agent/page__view__findpost__p__1904317"]today.[/url]

Nope, this was another Michael, but great man also. :)

^ I stand corrected.

Its quite useful though (user agent showing) I have already identified about 6 new bots and would be redirecting them lo-fi skin.
Should be there as default.