Forgive me for asking a career related question - I know that they can be frowned upon here, but I think that this one is general enough to be useful to many people.

My question is: How easy/difficult is it to get a job using language/frameowork B, when your current job uses language/framework A?

e.g. If you use C#/ASP.NET in your current job, how difficult would it be to get a job using python/django, or PHP/Zend, or whatever (the specifics of the example don't matter).

Relatedly, if you work in client side scripting, but perhaps work on server-side projects in your own time, how difficult would it be to switch to server-side professionally?

So, to sum up, does the choice of which languages/frameworks use at work tend to box you in professionally?