A list of Entity Framework providers for various databases
DB2 will work, but if you want to use some of the out-of-the-box .NET features that we all take for granted (think membership and role providers), you will have quite a bit of work on your hands. DB2 doesn't really do well with Stored Procedures, so you are also looking at a lot of parameterized inline SQL.
I also had some performance issues with the provider from IBM. There is another provider available from a company called DataDirect. It looks to be more performant, but it will cost you. For something like an external facing website, where speed is important, the cost probably will not be an issue.
I am sure that DB2 is a great system if you are building on another platform, but it wasn’t really designed to play nice with .NET. Unless you are already married to DB2, I would look elsewhere.
I don't think DB2 works with EF 4 yet. If it does, someone please correct me and tell me how its done. I cannot get the DB2 providers to show up in the Data Source dialogs.