The OED, which typically offers little or no meta-discussion about proper usage says that "a while" is

Improperly written together when there is no unification of sense and while is purely a noun.

Basically true to form, however, Merriam-Webster notices that too many competent authors and publishers make the "mistake" so frequently that the idea that it is improper in the first place must be questioned. Merriam-Webster (1996) says:

Although considered a solecism by many commentators [that is, an ungrammatical, improper, or otherwise frowned-upon use], "awhile," like several other adverbs of time and place, is often used as the object of a preposition <for awhile there is a silence — Lord Dunsany>.

So, as with many things, I think we take Merriam-Webster's commentary to justify the idea that on the grand scale of grammatical flubs, this one does not rise to the level of patently unacceptable or the obvious result of a lack of discipline. One of the OED's own examples of improper use is from W.B. Yeats. So, obviously, even the OED acknowledges great ones make this 'error.'

Bottom line: Yes, it should be "a while." Whether it is acceptable depends entirely on the degree to which strict conformance to authoritative convention is encouraged or required.