Strengh I currently adds 1 damage point to attack, when it used to be 3. Strenght II adds 2.3 damage points according to new tooltips, 2 from my experimentation, when it should be 6.

To replicate:

Open LAN world
/gamerule naturalRegeneration false
Player 1 uses Strenght I and punchs Player 2
Player 2 notices 2 damage points are gone from health bar.

Heal

Player 1 uses Strenght II and punchs Player 2
Player 2 notices 3 damage points are gone from health bar.

According with Dinnerbone this is an unintended nerf:

https://twitter.com/Dinnerbone/status/343175666604339200

Being so it should be fixed prior to official 1.6 release.