I'm of 2 minds: If the expired domains are somewhat related to the site you are driving traffic to.. No problem. This is done all the time with doorway sites.

If the domains are completely unrelated, then we ar in a grey area. For generic names, I would think it's ok. But using a site, say auto.com, to drive traffic to a site, sexywear.com, is pushing the limits quire a bit.

Just my opinion.

Harry