<p>Is it more performant to have a bidimensional array (<code>type[,]</code>) or an array of arrays (<code>type[][]</code>) in C#?</p>

<p>Particularly for initial allocation and item access</p>

I believe that [,] can allocate one contiguous chunk of memory, while [][] is N+1 chunk allocations where N is the size of the first dimension. So I would guess that [,] is faster on initial allocation.
Access is probably about the same, except that [][] would involve one extra dereference. Unless you're in an exceptionally tight loop it's probably a wash. Now, if you're doing something like image processing where you are referencing
between
rows rather than traversing row by row, locality of reference will play a big factor and [,] will probably edge out [][] depending on your cache size.
As Marc Gravell mentioned, usage is key to evaluating the performance...