I dated and married a colleague.. and after changing company we are still colleagues. We want to remain like that, if we do not decide to divorce ;-)
And hey, it's fun. Didn't take toll on productivity or anything. You get much more things to talk about if you are in the same office. And I think companies like that too (atleast in software, they do). It means longer commitment to the company.
Well, working seperately can be done anytime. But we are enjoying working together while we have the opportunity.