I dont have much personal experience but it's something I researched a while ago. My nephew threw a tanty in Target, embarassing my SIL. My brother took him out of the store for a while, then brought him back to apologise to me and SIL. I asked him what he was sorry for and he tearfully protested 'I dont know!'. He was 4 at the time and we wanted a Batman figurine. It interested me from a child psychology perspective. Anyway, research shows that at that age, a forced apology doesnt change behaviour from unacceptable to acceptable. They dont have the capacity to feel sorry if they aren't.

Here's a link to a blog entry:
http://www.janetlansbury.com/2009/12/youll-be-sorry/

I also found research on it but the site had commercial links so I cant post. If you google "child development apology", there's some interesting reading.
I love love love Janet lansburys blog!!! I read it a lot!!

It's true that for young kids it is actually hard for them to say sorry as they can't understand the meaning behind it well enough even if we explain it.

In saying that though, I still model it and always like DD1 to say sorry where possible but never force her to.
I sometimes say "when you are ready can you please say your sorry?" and sometimes 10mins after she will

If DD1 accidentally hurts DD2, she will automatically say 'oh sorry bubba'