You must have a rating to compete for any class prizes, or in sections with ratings limits. But you can play for the top prizes only in the open sections as an unrated player.
looks like you are quite right there, estragon. the qualification to compete in any except the open section or over 2400 section for the top prize money is to have completed over 26 games.
see: http://main.uschess.org/index.php?option=com_wrapper&Itemid=199
If you intend to play in rated tournaments in the USA, you might as well join USCF. If you are located in any other country, there are no USCF tournaments I'm aware of held in other countries, so there would be little benefit to joining USCF for those living overseas.
HOWEVER: if you live in Germany, and want to play for the big money in the World Open or New York Open (held in the USA) then yes, you would need to join USCF and somehow get in the minimum number of USCF rated games in order to not be a 'provisional' player. The minimum number of provisional games used to be a player's first 20 USCF rated games, but it might have been raised to 25, I'm not sure offhand. Of course, if you have an established FIDE Rating, I believe they accept proof of that (they look on the latest FIDE listing) to validate qualifying to compete for the big money, $10,000 or $15,000 in the top sections.
Any player who has no established FIDE rating, or has not completed the required number of provisional USCF rated games, can not compete for the big money.
Hope that helps you understand better...
Regards,
You must have a rating to compete for any class prizes, or in sections with ratings limits. But you can play for the top prizes only in the open sections as an unrated player.
You're only unrated the first tournament. Joining the USCF for one tournament would be cost ineffective. The reason unrated's don't get class prizes, besides the sandbaggers and cheaters, is they aren't in that class. After all, each class is for a specified rating range.