As far as I'm aware, without the certificate the data cannot be decrypted at all.
Do you have an older backup that still has the certificate and master key in? If so, restore it elsewhere and you can get at the data, at least. Not sure if you'll be able to copy the certificate and key over.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability