if you mean adsense ads, you can not put them on a page without any content. but you are talking about search box. i think it's not against the tos
I would read terms over real good. I personaly do not think i would do it. I think all the domain parking sites would have adsense ads or search box on them if it is not against googles terms.
You can add them in 404 page but make sure you have good amount of page indexed. Thats Search box only make those come in search which are indexed. On the contrary its always better to have default blog search enabled. People will get to the page they want to see. Display your add there . This will give u better money.
You can display Google Adsense on 404 pages. Google for search, I would assume the same thing, but better check out first. Peace,
You can and they do, just google for "Domain park adsense". You cannot place adsense ads on pages with no content, this includes 404's. As for the search, as google completly controls the search (so long as its not an in-site search) you can put them on any page where a competing search box does not appear. Pierce