Arkov

... As far as the Tumble skill goes, for every 5 skill points you put into this skill you also get a +1 bonus to your AC, and again, I believe, this is a DODGE bonus, not a Natural one. So if you have 10 ranks in Tumble, you would gain a +2 bonus to your AC

A slight note, which has been mentioned before but bears repeating:

Presently, if you have put 10 ranks in Tumble, you will get a +2 bonus to your AC even if your actual effective Tumble score is negative (due to, for example, wearing full plate, or using a tower shield). Do not expect this to remain the case forever. At some point, this will be changed so that the AC bonus from Tumble only applies when you have the effective skill ranks to back it up -- so a Fighter/Rogue wearing full plate and carrying a tower shield, and thus having a large penalty to Tumble putting him below zero effective ranks, will no longer get an AC bonus from his base ranks in Tumble.

As of v616, Tumble AC is now calculated based on the lower of your effective ranks and your base ranks, rather than only on your base ranks. This change will affect your character if and only if:

1) you have 5+ ranks in Tumble, and 2) you have significantly fewer effective ranks in Tumble than you do base ranks (likely as a result of wearing medium/heavy armor).