There are two Atom platforms supported by the embedded group at Intel: The Intel Atom N270 paired with the Mobile Intel 945GSE Express Chipset (Intel 82945GSE GMCH + ICH7M), and the Intel Atom Processor Z5xx Series paired with the Intel System Controller Hub US15W. Both solutions provide PCI Express ports that you can use to connect any PCI Express compliant LAN device.
One particularly suitable for Atom designs is the Intel 82574 because of it's relatively low cost, small footprint, and availability in industrial temperature skus. More information can be found on that product at developer.intel.com (search for 82574). Note that this provides GbE functionality.
Additionally, the ICH7M has an integrated LAN controller requiring just an external PHY to implement (such as the Intel 82559 Fast Ethernet Controller). Again, more details can be found on developer.intel.com by searching on the part number.
Further confidential design related information can be obtained if you have a non-disclosure agreement with Intel (you can apply through this site) by an Intel representative (you can request that an Intel representative contact you through this site if you don't have one already).
There are also a number of Atom-based off-the-shelf solutions available from member companies of the Intel Embedded and Communications Alliance. These include reference platforms and commercial production modules and boards. Do you plan to design a custom board or would you consider using one that's already available? If so if you could let us know the form factor and any other special requirements (e.g. industrial temperature, etc.) we'll be happy to guide you to the source.