I asked my friend who works at a T-Mobile store as a Customer Service Rep. This is what he told me:
You first have to get your Sidekick unlocked. Just bring it into a T-Mobile store to get it unlocked or a small retailer/service center who can unlock phones for just $10 to $15. The answer is yes, it'll work in India, AFTER you unlock it
However, I must add - all those features you like about your Sidekick, like picture messaging, AIM, facebook, etc will NOT work in India. The phone's settings make it nearly impossible to use these functions without T-Mobile. Unfortunately there's no current work around.
Also, the Sidekick uses built in memory. You will NOT be able to add memory to it. It's just a function that isn't available on any version of the Sidekick.
Hope that helps.