So it's not "Harry Potter & the Sudden Appearance of Micah Moses" then? Well damn...
Harry Potter & the Deathly Hallows
I think it's a play on words to throw people off the obvious Godric's Hollow answer (where Harry's parents died), since it's spelt Hallow as in All Hallows' Eve (halloween). However, since it was Halloween when Harry's parents died it seems like the title is still referring to the same thing. The coincidence of them dying in Godric's HOLLOW and during All HALLOWS' EVE is too big in my opinion. Oh, and the word DEATHLY just makes me more sure. :P