Should someone stumble upon this issue, make sure to not have require-login="true" in the main pages.xml file.
Juan, without providing more details, nobody will respond to you.
Do you use caching of style and image resources? http://www.seamframework.org/Documentation/HTTPClientserverOptimizationStrategies
Provide more details on what styles do you use ? Richfaces components?
Thanks Marek, my answer was actually the solution to the issue. Yes, I was using Richfaces, it's a known issue (see Weird Richfaces / pages.xml behavior). We just have to use login-required in every per-view page.xml file instead of setting that in the global pages.xml.