Suddenly brain went dead!!

Not if the plan is top-heavy.

I found this thread useful.

so, what you're saying is that Keys may not defer anything in their 401(k) if they (as owners) do not want to make a top heavy contribution. This 401(k) plan consists only of deferrals - the owners have never deferred because plan is top heavy but they thought after GUST, etc. that they could defer without worrying about top heavy and only worrying about passing ADP/ACP tests. If they're wrong on this, is there something in the GUST/EGTRRA changes that you think would make them have this idea? Thanks in advance.

If the plan is top heavy, if a key employee makes employee deferral contribuions & if there are no other contributions other than deferrals, the top-heavy minimum is required.

EGTRRA made numerous changes to top-heavy rules, but the only provision that actually states a 401(k) plan isn't top-heavy is §613(d). That only applies to certain safe-harbor 401(k) plans. This isn't a safe harbor plan so §613(d) isn't applicable.