[Cyrustorz]

I don't understand, O is much more electronegative than Nitrogen... Shouldn't the dipole moment be greater?

CH3CHO even has more electrons, so it also has slightly more dispersion forces on top of the dipole moment, I would have thought the boiling point of ethanal would therefore be greater than cyanomethane, but it is the opposite... I don't understand at all! Could anyone help me?

[AdiDex]

Dipole moment of Acetonitrile is 3.92 where as Acetaldehyde's dipole moment is only 2.7 . Both of them almost have same molecular mass .
So Acetonitrile should have greater boiling point .
Now the question is why Acetonitrile has more dipole moment ?
You are missing one factor in this case , Geometry . Acetonitrile has linear geometry where as Acetaldehyde has Trigonal planar . Can you think now?? what were you missing earlier ??

[Cyrustorz]

it may be trigonal planar but the Oxygen is still on the end of the molecule, so how is that not still a large dipole moment for a very electronegative oxygen?

[AdiDex]

The angle between the dipoles of the C-CH₃ and C=O is 60° , but in case of C-CH3 and C≡N it is Zero .
we can't calculate exact solutions just by taking some facts . We can't say simply that in this case this factor must be dominated or this should not be .

This is true that acetonitrile has more dipole moment .
It is very difficult to prove that the factor of electronegativity is being suppressed by the factor of Addition of vectors. But we have final results .

[Enthalpy]

Cyclobutanone has only 2.99D moment and, at least, has its CO aligned with the general molecule's axis.

Could it be that three bonds in CN are more easily polarized than two in CO?

I've searched for "partial charge" in nitriles and aldehydes but didn't find any page in a reasonable time.