My way is easier if you have good experience with the pen tool.
First, make your triangle. In my case, I just made a box, deleted 1 anchor point, joined the two points creating a segment opposite to the middle point and rotated about 45 degrees (please correct me if I'm wrong).
Next, hit the convert path to rounded corner button. I'm pretty sure this feature is already in CS5. Make sure you have the point selected when doing this.
Do this one by one for each of your points. The one in the middle should be the easiest one to convert. The one at the top and the bottom are a bit tricky but nothing a pen tool can't fix :)
As you go each of the top and the bottom points, with your Direct Selection Tool active, grab the furthest to the left angle for the point being selected and while holding Shift, bring it to the center creating a nice straight side but still rounded corners.
It should look something like this:
Again, do the same thing for the bottom and you will get this:
I hope this helps! Please let me know if I confuse you. I'll be glad to clarify myself some more.