Do subfloors in bathrooms require any special sealant or moisture barrier prior to adding standard floor covering (like tile)?
I think a quality bathroom installation includes waterproofing and an isolation membrane. I'm a fan of Schluter's Ditra underlayment, but there are other good options out there. A typical installation has 1/4" backer board glued down with thinset and screwed down, the isolation membrane installed with thinset (and taped if necessary), and then the tile installed on top of that. A membrane like Ditra serves both as waterproofing and as an isolation membrane; it lets the subfloor move slightly without transferring the movement to the tile, preventing cracking. Make sure to read the installation instructions (found on the manufacturer's website) carefully; with Ditra, for example, you use latex-modified thinset in some cases and normal thinset in others.