.Net MVC3 app_offline.htm issues with css/images

I didn't even know app_offline.htm worked for MVC :-)

As an alternative, you could create a global filter or if you have a common controller base class that when the maintenance flag is turned on, redirect all traffic to the maintenance view (flag can be in DB or app config).

If it's not that much images in design, you can embed css and images straight into the app_offline.htm
for css just use a <style> element.
for images read up http://www.websiteoptimization.com/speed/tweak/inline-images/