with the PS3 at least, the one i have experience with, its officially allowed to download your PSN purchases on up to 5 different systems, as long as you had your PSN account active on them, and they could be used by any other profile on the system as well. it used to be that you could share stuff with friends this way, like you buy a DLC and let your friend download it free, or vice versa. but seems the TOS now limits this to 'you, your children, and children for whom you are a legal guardian'. anyways, i have also read that some content does not work like this. and then there are other codes, like the ones in got with Dragon Age, which werent redeemed through PSN but through Bioware, and tied solely to the one account.

so will they be PSN codes?
and will both me and my girlfriend be able to use the skins/tattoos/etc since she has her own profile?

With xbox it's associated to profile, so in theory if you can recover your tag to her system then transfer saves.....I'm not really sure.

well, we own both an xbox and a ps3.
we have so far only ever purchased games, no dlc. and i think the only bonus codes i have used were dragon age.
on the PS3, all purchases have been made on my account, and she is able to use them.
on the Xbox, all purchases have been made with her account, and i am still able to use them.
so yeah, in theory it should work. but like i said, i have read that some content doesnt work like that, and i have no experience with bonus codes.

On the Xbox 360 purchases are registered to the Gamertag that made the purchase, and the serial number of the console used to make the purchase. So in order to share, Person A would have to recover Person B's Gamertag prior to the purchase, and then Person A (now using Person B's account) would make the purchase himself. Person B would then only be able to use that content when connected to LIVE on that profile, while Person A could use it on any account while using that console.

On PS3, that depends on the specific download.

The "standard" rule is up to 5 profiles can have access to it, but some DLC has been profile-restricted, and some downloads will allow use with more or less than that figure.

As this relates to Sony's TOS, I'd suggest calling them and asking how it works. Discussing ways for other folks to get the content for free isn't allowed here..