>would you expect your significant other to post that you guys are 'in a relationship' and post up 'lovey dovey' pictures on social networking sites like myspace, facebook etc.?

Whether I would expect it or not depends on that person's character. I would certainly not appreciate or allow it,...

>and for those of you who don't/won't do so, similarly, what's your reason and mentality behind this?

1 - It's my private life, I don't need to advertise it to the world (that's taken care of by marriage, if needed).

2 - I also don't like to post pictures of me on the internet, since you never know if that will be used against you in the future.

3 - Relationships do end, and it's probably impossible to remove all the information from the internet, once it has been posted. I don't want to have a public record of my past relationships that anyone can check (can you imagine in twenty years having your kids ask you who was the girl you were kissing in whatever random picture?)