A couple of things to keep in mind...depending on the rack you own, there are some better and not so better solutions.

Reflector Mount Bolts - if you have basic reflector mounts then I would say try to find one of the older CatEye Reflex or TL-LD500 taillights. They are the only one that are a real reflector and also pretty good flashing lights. I like the idea of a passive ( real reflector ) aspect of the light.

The other thing to keep in mind is that a lot of the rack companies are moving towards a mounting system that is standard in Europe. This has a two bolt mount that fits horizontally on the rack. I know some light companies are also making mounts for their lights that fit this new rack system.

I use a new version of the Reflex with this...http://www.cateye.com/store/parts.php?cid=3_30

I wish all this would be standard and we would not need to have adaptors!