Last month I reported on some issues that Google had raised about using different types of redirects to park domains. It seems that the best way to park domains is to change the nameservers (such as ns1.sedoparking.com or ns1.parked.com) for the domain. Micheal Gilmour had a great article about this issue on his blog. He puts this move by Google in the context of their recent moves to cut payouts to domain owners. He also reported that his ParkLogic solution for large portfolio owners does allow dns switching - so his model still works.
We also get some hints that Yahoo may be blocking some domain redirects. (This from the members only section of the Parked forum, so no quotes.) The best advice for both Yahoo-based services (such as Parked.com, WhyPark.com, and SmartName.com) and Google-based services (such as DomainSponsor, Sedo, and NameDrive) is to set the appropriate nameservers at your registrar. If you use a service such as ParkLogic to send traffic to domains - be sure that this service is actually using the dns to send the traffic rather than some other approach.