I remember hearing somebody (Douglas Rushkoff, I think) suggest that ADD-like behaviors might be an adaptation to a childhood in which one is inundated with aggressive, flashy advertising. Adapting to reflexively ignoring things that are designed to steal your focus probably affects your brain in the long term, much like meditating regularly can.
Interestingly, I find that this tends to affect the men of my family more than the women. My mom, sister, and wife don't seem to have this problem at all. While both my dad and brother all suffer from an inability to hold a conversation if the TV is on.
Of course, that's a pretty small sample set.
I have also found that a daily time of quiet prayer and meditation is helpful. If you don't believe in prayer, some other kind of meditation will probably help, too. Lately I've been incorporating MP3s from pray-as-you-go.org, which are very well done short sessions of Ignatian guided meditation incorporating beautiful music.
I'm pretty helpless with group conversations, too. I guess conversation uses a lot of different aspects of attention, so you can't just fire off background tasks for listening to music and each conversation.
As for the people who can do concurrent conversations -- I think they're very good at filling in the gaps of each conversation that they miss. If each conversation is mostly small talk and little stories, you can lose a few words from each sentence and still follow along, especially if you can pick up on voice inflection and body language in your background tasks (as our more socially adept girlfriends and sisters seem to).
I find myself to be much the opposite. I have trouble focusing on on any one conversation, I'm too easily distracted. I can't hold a phone call or conversation while a TV is playing at similar volume. In a restaurant, I feel like I hear all conversations and the music, and I can't seem to shut them out. It's not that I'm listening to all the conversations, but I hear them, and it makes focusing on the conversation I'm trying to hold difficult.
With programming and reading, however, I do find that I can shut out background noise, and I suspect this is because it's a different kind of thinking than what distractions around would be causing.
I suspect that because the attention is given to a mode of thought (reading, programming), and that that mode is not competing with other forms of the same mode (I'm not trying to read two books simultaneously), that this enables someone (or maybe just me, I dunno) to shut off the other senses. While with listening and hearing multiple conversations, it's like trying to read and having random words getting thrown into the sentences you're reading.