The Huffington Post notes: “Brad Pitt was on hand at Saturday night’s Producer’s Guild Awards, in part to support his partner Angelina Jolie…” That partner caught my eye–specifically as a way to denote someone’s opposite-sex, longterm and, all things being equal, permanent love interest. My observation is that this has long been common in the UK–among both unmarried and married couples. The former makes sense because of the lack of suitably dignified terms. (Girlfriend? Lover? Main squeeze?) The latter case is more interesting. I hear it as an implicit disapproval of the traditional husband, wife and spouse, which presumably bring with them whiffs of an oppressive heteronormative (thanks, Elizabeth Yagoda) patriarchy.
The Oxford English Dictionary’s note about partner supports this political interpretation:
Now increasingly used in legal and contractual contexts to refer to a member of a couple in a long-standing relationship of any kind, so as to give equal recognition to marriage, cohabitation, same-sex relationships, etc.
Anyway, opposite-sex amorous partner is on the rise in the U.S. Just in the last few days, there has been:
Denver Post, January 24: “[Jim Jesperson] is preceded in death by his partner, Erin O’Niell…”
notoneoffbritishisms.com 
Among other things the expression replaced the dreadful “common law wife” here in the British Isles, and how glad we were when it did.
…especially as there was never any such thing as a common-law wife or husband. Living together gives you no legal rights, even if you’re called a “partner”.
While it’s true “common law wife” was a fallacy, note English law on the issue of partnerships. These quotes are from the British Government website:
“There are over four million couples living together in England and Wales in cohabitation, and they are given legal protection in several areas.”
and:
“Same-sex couples can have their relationships legally recognised as ‘civil partnerships’. .Civil partners must be treated the same as married couples on a wide range of legal matters.”
I seem to recall that common-law wives/husbands had (or maybe still have) some sort of legal status in the U.S., acquired after cohabiting for some set number of years.
They do under Scottish law too I believe, which is quite separate from English law. (A legacy of a once independent scotland, the legal system and church authorities were never integrated into those of England).
It varies by state:
http://en.wikipedia.org/wiki/Common-law_marriage
In the US, “partner” is not restricted to *opposite* sex love interests, probably because the possibility of gay marriage was for so long seen as an impossibility.
It is not restricted to heterosexual partnerships in the UK either; it can describe any couple and is the usual term for someone to refer to their same-sex love interest (although husband/wife are becoming more common since civil partnerships were enacted).
I understand from the article that it is just less likely to refer to an opposite sex relationship in the US than in the UK. The main thing it denotes in the UK is a long term relationship. I remember my mother insisting that my boyfriend now be referred to as my “partner” after we moved in together.