I'm creating a site column and I'm wondering if it's possible to change the
default value to use a calculated value? I know it's possible to do this via
the UI, but I need to be able to do this from a feature. I'm open to using the
object model via a feature receiver if needed.

Here's what I'm starting with:

<Field ID="{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}"
Name="ExpireDate"
Group="Custom Columns"
Type="DateTime"
DisplayName="Expires"
Format="DateTime"
SourceID="http://schemas.microsoft.com/sharepoint/v3/fields"
StaticName="ExpireDate"
>
<Default>[today]</Default>
</Field>