Though there is little or no evidence at this point that the explosions in Boston today are part of some broader, much less national, pattern, authorities are hastening to provide additional security at large-event venues in other cities (especially New York).
Meantime, the best online resource for tracking events in Boston is probably the Globe‘s liveblog.
We’re at the point in most incidents like this where anything seems possible (as most Americans probably remember from the wild reports and wilder fears on 9/11, which was horrific enough as it was). If you live in Boston, stay home until things are better sorted out. If you live elsewhere, try to avoid jumping to any conclusions. We should know a lot more pretty soon.