I've talked to people numerous times who've mentioned that they'd like to have something up which is pro-atheism in the office but they "know" that as soon as they do they'll be told to take it down by a theistic boss or complaints will roll in from coworkers, etc.
I know many people on here probably know this already but simply for any who don't... they can't. It's considered religious discrimination. Now an employer CAN say "no religious items" potentially. THIS however I'm not 100% sure of but I've been told by 2 attorney friends that if they're items not necessary for the practice of faith, such as a yamaka (head covering) for a someone of the jewish faith, but merely "decorative" then an employer has the right to say no 'decorations' in the office/cubicles/etc.
They may NOT, however, say "No crucifixes but statues of Buddha are acceptable." IE: all or nothing.
Here is the EOCC definition as well as the link to their page:
EEOC compliance guidelines
"Religious beliefs include theistic beliefs as well as non-theistic
“moral or ethical beliefs as to what is right and wrong which are
sincerely held with the strength of traditional religious
views.” Although courts generally resolve doubts about
particular beliefs in favor of finding that they are religious,
beliefs are not protected merely because they are strongly held.
Rather, religion typically concerns “ultimate ideas” about
“life, purpose, and death.” Social, political, or economic
philosophies, as well as mere personal preferences, are not
“religious” beliefs protected by Title VII."
Now, of course, this doesn't mean that a vengeful boss couldn't find some OTHER excuse to fire somebody who draws their ire, just as they always can in other situations where they may not legally "discriminate" but instead find a time the person was 2 minutes late or what have you as an excuse for termination.
If you're willing to stand up to that chance, however, it's always fun to see their face when you explain that your NPR "What I Believe" poster featuring Penn Gillette's "There is NO God" speech is as protected as your neighbor's cross, lol.
NB: I am NOT an attorney, these are only my opinions and not to be considered as legal advice.