playing with意思就是修改。I'm still playing with it的意思就是还没最后定下来，还在改进，修改

rule out
1. To prevent; preclude: The snowstorm ruled out their weekly meeting.
2. To remove from consideration; exclude: The option of starting over has been ruled out.
http://www.thefreedictionary.com/rule out

play with
vb (intr, preposition)
1. to consider without giving deep thought to or coming to a conclusion concerning we're playing with the idea of emigrating
2. to behave carelessly with to play with a girl's affections
3. to fiddle or mess about with he's just playing with his food
http://www.thefreedictionary.com/play with

因此我认为
we have ruled out ***playing with the final go-live date of Jan. 3, 2012
可paraphrase为：
we have considered not to make any change about changing the final go-live date of Jan. 3, 2012 any more.
换句话说，
我们的交货和预算很灵活，但是最终启用日期已经确定是1月3日，不会再更改。
翻译：
我们已不再考虑更改（已确定）最终启用日期，即2012年1月3日。

play的具象意义尽量体现出来；“发布”即含有“最后发布”之意，没有必要一个萝卜一个坑地翻译出来