Taken from YouTube's FAQ...
Depending on site traffic, changes to video information can take 8
hours or more to show up in the search index after they have been
uploaded, changed, or removed. This includes changes to tags, ratings,
views, and comments. Under extreme site traffic we have seen some
videos take over 24 hours to index. To bypass this time you may want
to send your friends the link of the video via email or private
message.
Note that unless the user name of the uploader is specified as a tag
it may not be indexed for a video search. To find a user (and their
videos) perform a search for their user name select "Channels" on the
left side of the search results.