This exception message has caused us problems. Long story made short, we see this with a remote .net assembly running under IIS 6.0 on a 2003 Server with SP1 and the calling client is an XP machine. It seems SP1 installs a never version of System.Data.dll. There is a hot fix for this, though the hotfix does not resolve the issue, it only gives the calling client a more meaningful message.
http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=106937&SiteID=1
If you are using .NET Remoting on a 2003 Server, do not install SP1 until you know it's safe.