If you're using a custom web part to login, I would guess that that's where the problem lies. Most tutorials out there show building a custom login web part using the ASP.Net login control. Although this works in SharePoint 2007, this will not work in
2010 without major customization. Use reflector on the SharePoint login page and you'll see what I mean. They are using the ASP.NET login control, but much of the functionality has been overridden.
If you just want to log somebody in with a user and password, you can find the code to do that in membershiprequest.cs. Essentially the magic line to do that is:
Microsoft.SharePoint.IdentityModel.SPClaimsUtility.AuthenticateFormsUser(new Uri(web.Url), request.UserName, request.Password);
The login web part is planned, not for the next release, but the one after that. It's probably 4-6 months out.