Here is the answer I provided for someone else on that question 3 days ago:
My understanding is that there are vfx houses using version 2.5 of shake still because it is powerful and the last version to run on a pc and a mac.
Learning Shake version 4.1 is still a great idea. It is a work horse and extremly strong compositing application. I reccomend moving forward with Shake as most production houses will still use it for some time.
..... also rumored that apple and/ or old developers of shake (nothing real) are working on a new compositing package that runs on a faster or smarter code more similar to Motion, with the compositing likeness of Shake.
Other programs that may be of interest are Combustion and After Effects. The transition from Shake to Combustion is very easy once you know Shake.
Hope I helped answer your question. Stayed tuned for more advice from other people here on fxshare.