I completed a draft of the "Guidelines for the use of Stack Exchange Network Logos and Names" this past weekend.
##Update
Here are the relevant portions from the final draft of the logo usage guidelines. Pending any last-minute changes, this should pretty much cover "fair use" of the logos in your applications.
Design of Your Product
Do:
- Do design your product with unique branding and logos.
- Do feel free to use our names or logos for the purpose of labeling our sites within your product, as long as use of such logos could not be confused with the branding or endorsement of the product itself.
Do not:
- Do not copy the look and feel of our products.
- Do not include elements of our network that are not user-contributed content (i.e. no copyright material).
- Do not use our name, logo, or service to promote or allow fraudulent, abusive, or otherwise illegal activities.