Based on the playtest docs, there's no reason why DFRPG can't be played in other worlds. There's just not a strong case as to why you would want to do that.
DFRPG has a certain flavor, and it's designed to fit the style of the DF novels. Put the same characters in Middle-earth (for example) and it totally changes the feel. There are plenty of RPGs out there designed to handle Middle-earth (or whereever) and DFRPG would require a lot of conversion.
Spirit of the Century is the same way -- it's built on a "pulp" feel and would handle a cinematic campaign much better than a gritty one, in my opinion. The rules are constructed to allow for characters to be more powerful than many other systems.
DFRPG (or SotC or FATE or whatever) works well for superhero gaming, pulp gaming, cinematic action-movie gaming, and so on. Not so good at handling worlds where characters are more limited in what they can do.
Not saying it can't be done, but the styles would probably have to match somewhat.