Hey- it's the THOUGHT that counts! While roses are very traditional, any pretty bouquet will do. Make sure they're fresh so they'll last at least through the first weekend of performances. It would also be a good idea to arrange for a small vase to hold the flowers so she can keep them nearby in the dressing room.

Sometimes it depends on the show. When my girlfriend's sister was in "The Secret Garden," for instance, I got her lilies (Lily is a character in the show). We did "Benny's Four Rooms" and all got paper daisies (which are talked about ad nauseum in the show.) If you don't know the show very well, or there's no cute tie-in, then roses are always good. Tulips are always a bonus, too, because no one ever gets tulips.