If i where to use a brute force(Brutus) on a target UNIX system, would someone be able to find out, and if so is it likley, if so, would they do anything???

Yes the administrator will probably see your connection attempts and he may contact 'authorities'...

theres couple of steps before u attack a computer, network, site etc...

First off You should Fingerprint them and learn as much as possible
http://packetstormsecurity.nl/papers/general/Footprinting-faq-v0_1.txt

U should connect to gateways so you wont get caught that easly

when u actually do get in, always remove EVERY trace possible so u wont get caught

here are to GREAT papers written by Van Hauser of The Hackers Choice (THC) group

http://www.thc.org/papers/COVER-1.TXT
http://www.thc.org/papers/COVER-2.TXT

Remember never go in blind, or you will prb get caught and blamed for thousands or millions of dollors worth of damages because u simply loged in.

+++EDITED++++
before u start bruteforcing make sure u have a nice big wordlist so you actually have a chance of breaking user/password
heres a link of A LOT of wordlists

http://www.phreak.org/html/wordlists.shtml

"would they do anything??? " depends what country and how mutch stuff u fuck up, anyway the police will only get involved if u fuck up alot of things.

Brute forcing remotely is rather painful and long..
why not just find a vulnrability every box has a vulnrability at some stage :p whether it be the owner "social engineering" or a software flaw.. It's alrite for thing's like hotmail etc but hey..