[chrismonsanto]

re 1) one of the primary limitations of the lambda calculus wrt concurrency is that it does not model time--there is no way to determine how long a computation takes. An example of a function the lambda calculus cannot define is f(x, y) where the fastest computation of x and y wins. You'll see that function pop up in a number of places in the Haskell world, where it is called "amb". This function is obviously important from a pragmatic POV but it turns out it is also important when doing formal work as well: see the parallel-or/full abstraction problem for PCF[0].

My personal favorite concurrency formalism is the Chemical Abstract Machine[1]. The y-calculus is a neat extension that preserves the spirit of the lambda calculus, but also permits (purely non-deterministic) concurrency.

[0]: http://en.wikipedia.org/wiki/Fully_abstract#Abstraction

[1]: http://www.lix.polytechnique.fr/~fvalenci/papers/cham.pdf

[mafribe]

Most process calculi (including the CHAM) don't model time. I think it's perfectly possible to consider computation without timing.

Adding timing is straightforward for any model of computation with operational semantics. This addition generally changes the semantics dramatically (eg. equalities that hold). There are many timed versions of process calculus, e.g. timed CCS, timed CSP, timed pi-calculus.