I for one consider Haskell to be my secret weapon - that tool that gives me a little edge over other developers, and frequently find places to used it in my consulting jobs.
DSLs, for example, are pretty hot these days in custom software. Haskell is a ridiculously good fit for this type of work.
If that isn't the kind of "commercial" environment you have in mind, some kind of polyglot project is probably best (I think good UI is one of Haskell's weaknesses). I have a project going right now that has Haskell running as a fast cgi "service" in the back end (I tend not to like 'frameworks' too much, and really don't like the ones available for Haskell right now), served up by a Flex (Adobe Flash Platform) front end. The eventual intent for this project will be to be sold commercially* to smaller-to-medium sized businesses as a financial planning aid.
Last thing I'll say is that, while I personally love Haskell, I think its FP in general that provides the biggest direct benefit. To this end, if you find yourself eyeing another FP language such as F#, Scala or Erlang, and think it'll be a better fit then I say run with it.
*Actually, the project will be used internally for at least a year before marketting to outside parties begins.
