Movie spin-offs aren't all that rare, but still they don't happen ever day. Usually because they don't work. Most spin-off's take the most popular characters from a series (I.E. X-Men Origins: Wolverine) and don't know what to do with them. Sometimes, they take a popular background character that exists only to provide comic relief and never really have anything to do with the story... and give them there own movie. So, ya, The Minions from Despicable Me are getting a spin-off.
This probably won't work. The minions are comic relief (And the don't even talk), so it's hard to see them being able to star in their own 90-minute movie. This comes after the announcement of the much more logical The Penguins of Madagascar spin-off film being released in 2015. They can talk, have had their own successful (And funny) TV series and above all else, can actually have their own story. I just don't see the minions being able to tell a good story. The spin-off (Which will probably be called Despicable Minions) is set to be release in 2014.