Hyp, I've gotta tell ya, the PHB description of the monk is just crap. Back in 3.0, it said "it makes no sense for a monk attacking unarmed to have an off-hand attack", and one paragraph later, pointed out that monks can make off-hand unarmed attacks along with one-handed weapons (and the 3.0 FAQ had to clarify that they could do it along with two-handed weapons as well). Even the RAW can be self-contradictory.
As it stands, the 3.5 description of monk unarmed strikes only mentions the off-hand for one purpose: to point out that a monk with his hands full (either with weapons or any other objects) can still make primary unarmed strikes. If a fighter has a sword in his primary hand, any unarmed attack he makes is off-hand, -4 to hit and 1/2 Str bonus to damage. A monk in the same situation can make a primary unarmed strike with any part of his body without suffering those penalties. The PHB text does *not* forbid off-hand unarmed attacks for monks who choose to make them (it'd be silly to think that a fighter who multiclasses monk forgets how to make off-hand unarmed attacks!), but merely points out that the monk can't be forced into making them the way other characters can. The FAQ clairifies this (as if common sodding snense weren't enough) and is even nice enough to remind us that an off-hand unarmed strike, even for a monk, takes half the Str bonus to damage (even though the poorly worded PHB text might suggest otherwise).