I'm coming towards the end of the story; infact I'm just about to face Seren for the last time. 
I've played through the game before, but decided to revisit it to get a feel for the universe before the sequel. 
Here's my dilemma: 
 
On this playthrough I decided to save Kaiden; in fact I've never not saved Kaiden, but now I'm regretting it and want to play through again so I can save Ashley. But that's all I want to to do. No side quest stuff or anything.  
 
So I guess my question to people who are willing to through their thoughts at this is: if I go for this second playthrough, and only do the main story will my previous decisions on my first playthrough outside of the main story carry over to Mass Effect 2? 
 
The reason I ask this is that the E3 demo guy explained that there were quite a few decisions that will carry over to the sequel, not just character development. 
   
Oh and also, would anyone like to see some sort of chance to modify the look of your Shepard? I've got the typical RPG customisable aesthetic anxiety and really want to change a few of Shepard's features.
 

Giant Bomb; away!!  

Every time the credits roll, the game makes a "final save". These final saves are what you see whenever you make a new game+.
 
So no, I'm guessing the decisions made on two different playthroughs will not be able to affect the same ME2 playthrough.