I can provide two examples of non-optimized apps:
Password Safe is a for-fee Password Manager that looks and works great on my SmartPhone. However, on my Xoom, it does not scale AT ALL. It appears on the Xoom with huge black borders around the left, right and bottom edges, and visually is the same size as if it was displayed on the phone. It functions exactly as it should, but clearly is not scaling.
There there is
Andoku Sudoku, my favorite free Sudoku game for Android. On the Xoom, it properly scales the play ares, so the 9x9 grid expands perfectly with the top, left and right edges touching the end of the screen. However, the Sudoku "keypad" with keys 1-9, Clear, Undo, etc., are displayed below the grid in the original phone size, completely unscaled, with much wasted space around it.
Some apps definitely handle large screens better, and with the introduction of fragments for Honeycomb and now the libraries going back to 2.2 (?) that let fragments run in older versions of Android, I'm sure a lot of the developers are thinking of ways to redesign their apps to take advantage of the new options.