Yes, you can write C# code for your web-enabled templates. You just have to
make sure it uses the Office 2007 object model.
See here for details:
http://msdn2.microsoft.com/en-us/library/ms772259.aspx
Not all classes of the .NET framework will work when used in a web-enabled
form. (e.g., you cannot open a MessageBox using System.Windows.Forms)
Thanks,
Frank

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> If an infopath form is web-enabled inside a MOSS 2007 form library, am
> I still able to add my own C# code into the form like I am with the
> fat-client forms?
>
> ~john