I've had some modest success with exploring them as kind of a mostly-urban underclass, seen all over the place but often ignored or put into menial positions. They're a little bit informed by the general role of immigrant populations, a little bit informed by a kind of "stray mongrel" aesthetic. It leads to interesting ideas such as halfling domestics being popular, halflings sending money home to the family, halflings tending to be close-lipped around anyone not of the community, and even the idea that they might form clandestine "community welfare organizations" dedicated to the protection of halflings from the larger, stronger races... and of course, that's a common origin for organized crime syndicates. I realize the concept of "halfling Mafia" is a joke in greater gamer culture, but honestly some bits of Gangs of New York and some touches of historical yakuza, with a touch of Swords of Lankhmar for the idea of how covert urban warfare might play out, and it all works out pretty well.
So, closest to the 4e model, but with more of an emphasis on the fact that most people just don't pay them any attention, and they're fine with things that way. Sure, you can trust them to have your back in a fight or be solid traveling companions, but if they're garrulous, it's rarely stuff of substance. When halflings actually confide in you about stuff, then you know something is up.