Apple is everywhere in Hollywood
Just off the top of my head:
'CSI' uses them all the time
'UC Undercover' all the time
Target uses them in their ads
Countless movies already have the TiBook in them, 'Kate and Leopold' for example.
Plus, you see the TiBook randomly just about everywhere.
But they are almost always disguised, and the Apple on the back removed (SOMEHOW, must be camera tricks in post for film, or photoshop in the still images, obviously). You have to look carefully to realize they are PowerBooks. They are very stylish computers, and since only 5 percent or so of the population know about them, people who are watching the show think they are just fancy-schmancy 'government computers.'
On a side note, I always love it when they are using these computers, and 'zoom in' pre-recorded images from surveilance cameras, or 're-create' a scene digitally on a laptop in about 10 seconds. Cracks me up. Its so fake.