Last night, I learned that nucleophiles attack the carbon at ~107 degrees to the C=O bond axis. So, my curiosity took me to the original paper by Burgi and Dunitz.
I read the paper a few times and here is what I learned:
1. As the nucleophile attacks, the R groups bend and C-O bond becomes longer.
2. As the nucleophile approaches even more, RRCO becomes even more nonplanar. Looks like sp2 hybridized carbon becomes sp3 hybridized.
One of the R's in aldehydes is hyrdogen. This also explains why aldehydes are more reactive than ketones. They have less steric hindrance in the system. (This is one of the reasons.)
I think this is the way it happens:
From molecular orbital theory approach; the electrons in the HOMO of the nucleophile interact with the antibonding orbital (LUMO) of the C=O bond. Both the bonding and antibonding orbitals are occupied now. This breaks the Pi bond and the electrons move to the most electronegative atom (oxygen) in the bond.