While LCD TVs are more lightweight than their old, CRT counterparts, putting one directly on top of your DVD player, or any other electronics device, is probably not a good idea for a variety of reasons.
LCD TVs are smaller and more lightweight than ever before but all but the smallest ones probably still weigh enough to damage your DVD player. Many DVD players are small and lightweight as well and they cannot stand up to much pressure or weight.
DVD players, like all electronics, generate heat. This heat usually vents out of the player via a series of holes or slits found on the top of the player. If these vents are covered up by a television then the player could overheat and stop working. It would most likely void the player's warranty as well.
If space is at a premium for your home entertainment set-up, you may want to consider mounting your LCD TV on the wall and then placing your DVD player under it. Many companies make wall mounts for LCD TVs, which can make the mounting process easier.