The issue with officially supporting AIX 6.1 is the certification and verification from IBM. Although a lot of people are successfully running on AIX 6.1, Splunk (or any other company) can not claim that their software is certified to run on AIX until their get the green light from IBM.
With that, I will also say I haven't had issues running Splunk 4.x either.
I'm running an indexer and a couple dozen LWFs on AIX 6.1 as well. No problems. And I'll also add that tech support has always helped me, even knowing I'm on 6.1.
I can't comment on official support, but I am successfully running 4.1.x as a lightweight forwarder on AIX 6.1 TL04 with no issues.