ok i'll give you the down-low on b&s' from someone who attends them and likes them!!
yes you will get a lot of utes there, many of which are categorised as 'b&s utes' - these are the ones with big aerials, lights, stickers, bullbar etc... but there are many small cars there too..
usually you pay about $90-$110 for the night.. sounds expensive, but you get all your drinks (usually there's beer, vodka, rum, bourbon... and sometimes other things too, you get given a cup and can go back to the bar all night for re-fills), food (night and brekky), a band (or two!), and then u get stuff like hats/stubby holders/stickers etc.. depending on the b&s that you are at
as for sleeping with a random in the back of a ute - well yes that's always a possibility if you want to but more often than not i've gone back to my own swag so it's not like everyone hooks up with someone...
as for dress - they say it's black tie, but anything you wear will get ruined so don't wear anything good!! most girls make a skirt that goes to the ankle out of any fabric, with a singlet top (obviously not in winter), with jeans underneath and boots/boat shoes etc.. blokes just wear cheap black pants with a white shirt and bowtie (which may just consist of electrical tape)
it's tradition that most people will have food dye at a b&s, which you either put in your drink or in your mouth and spit/throw all over eachother - sounds gross but it's pretty funny to wake up covered in food dye and see the state of your clothes.. just a word of warning - don't put food dye on people you don't know cos some get a bit pissed off!!
and if you're a b&s virgin make sure you tell everyone cos they will give you presents!!
seriously - a b&s is pretty feral and a lot of people don't like them, but they are there for country people to catch up (when u live far away from your mates u can't just go out for coffee to catch up), and i'd say don't judge a b&s untill you go to one - be open minded, be prepared to get lots of food dye/alcohol all over you but most of all meet new people and have fun!!! (as you can probably tell i love them!)
there was a b&s in wagga last night (26/11) but there aren't really any in that area for the next few months... the only ones i know of are gular on 10/12, then a couple in january (walgett and tullamore - but those are both a fair way from wagga)