I'm about to buy someone's 09 Langster and I've calculated the BB height is 261mm (correct me if I'm wrong); that's lower than my road bike. At what BB height (and above) would you consider safe to ride fixed to avoid pedal strike?

Ah ****. Elite Poop.

BB height isn't an often used measurement because it fluctuates with tire and wheel size.
What is the bottom bracket drop? This is the distance the center of the bottom bracket sits below a horizontal line drawn through the wheel axles.

Its not a normal measurement for geometry, but it is the relevant factor for the question being asked.

It isn't regularly given with geometry charts so we don't really have a means of comparing it easily to other bikes.

You should check out this thread. It's pretty similar.
Which langster is this? one of the city themed ones?

Since you're in SF, I assume you're taking it to hellyer? Pedal strike is not an issue there.

Is there just a straight forward number answer? like 270mm or something? I calculated the bb height of the langster including the thickness of a 700 23c tire.

Not really. Depends where you're riding.

10.28 inches.

I see what you did there.

There can't be a simple number because there are more variables involved, like the q factor of the cranks, crank arm length, type of pedals and riding style.
The Langster is designed to be ridden fixed, if you're using run of the mill parts, it should be fine.

Any BB height can be ridden safely, but the lower the BB, the slower and/or wider you'll need to take corners to avoid pedal strike.

There is no particular BB height that is deemed unsafe. If you feel your BB height is too low, you can compensate by using large tires, shorter cranks, and or clipless pedals. Also you can adjust your riding style.

like most of the others have said, it's your riding style, not bottom bracket height, that will most greatly influence whether your smack your pedals on the ground. due to my own idiocy of not compensating for different geometries, i've managed to pedal strike on everything from road bikes to a hybrid. the hybrid was awesome for "omg, i think i'm going to kill myself" style strikes; i managed to do it at least once a week. just learn what your lean in limits are on the bike, and adjust speed and pedaling accordingly.

When I first started on a fixed gear bikes, before riding new bike, I would put the pedal at its lowest position and lean the bike over to see when it made contact. Supposedly I would somehow remember this angle when I was actually riding and avoid going past it.

What I did glean from this little experiment is how much difference there can be from bike to an other depending on the factors already mentioned here (tire size, crank length, pedal design etc.)

FWIW my 29er beach cruiser has a BB height of 12.5, rolls on 50mm tires and SPD pedals. That bike has an incredible amount of lean over before you ground a pedal. My Bianchi conversion has much less.