Are these compatible bottom brackets? I have a LeMond Big Sky SLT &amp; an Opus Triton. Every bikepedia-esque site I went on said they had incompatible bottom brackets.

When I actually read the brackets today the LeMond has a BSA. The Opus only states: BC1.37x24.

BSA has the same measurements: 1.37x24 (according to wikipedia.)

Does this mean the crank from one will fit in the other or am I missing something?

I've read a lot about all the crank differences and compatibility issues etc but frankly I can't identify what I have to know if they're different.
Matching BB is one of the hardest jobs as a paper exercise compared to eyeballing them side by side.  As suggest in @Batman comment, length and offset are both critical, yet most sellers only specify length. 

As far as matching Cranks go, there are a  number of systems - square taper was (an probably still is) the most common - there is some confusion over the compatibility between a couple of different standards, but it does not seem to be a problem in reality. Spline cranks come in a couple of different flavors, which are incompatible. Have a read of Sheldons webpage on the subject.