[dragonwriter]

It appears that federal employees are not generally restricted from outside employment, and only need a waiver to engage in certain official acts which would affect the financial interest of an outside employer or entity with which they are in employment negotiations.

http://www.justice.gov/jmd/ethics/generalf.htm

[specto]

Depends if they have clearance, if they do they must get approval first.

[dragonwriter]

Yeah, the "generally" in my post was not decorative: there are certainly all kinds of special conditions where more than the rules applicable to federal employees in general apply, and security clearances certainly come with their own special rules.