This download link might be from IBM, but these are “generic” reference drivers, which mean they will work with any Broadcom NetXtreme II based chipset.
The added benefit is that these seem to support Hyper-V better.
Problems with the Server 2008 inbox drivers are well documented here.
You can download the complete updated driver collection for Broadcom here.
Thanks go out to the comments in that referenced blog post for providing the IBM Broadcom link.