Anyway, this is a "paper cut" in the language that has been bugging me for a while, and since there's now a call for suggestions for Haskell 2014, I thought I'd ask about it.
I've also thought about this issue and I agree with Garrett, allowing that trailing comma (or semicolon) would help readability*. If it doesn't work with tuples, perhaps we could at least do it with lists and records?
* It also hurts source control diffs a bit, as adding extra commas will give diffs that suggest that one additional line was changed.