I'm with ya - your daughter will be fine despite the crying. I, too, used the Weissbluth book quite extensively.
We used to be able to put our son right in the crib for nap and he'd clock out. He then hit two years old and some tweaking on our part was in order.
Can you more closely mimic the night routine when you put her down for naps? What we had to do was read him the same books before the nap that we were reading to him at night. For him, it is all about the routine and by taking the 5 minutes to read him the books he 'got it' that is was nap time and that, just like when he went to bed, it was time to go to sleep but that mommy and daddy would be there.
I would avoid any return trips unless she is hysterical or the cries are different than just the "I'm bored, come back mommy!"
Good luck - I find sometimes that the tiniest of tweaks in the sleep system work wonders. Our son STILL at 2 1/2 years old takes a 3 hour nap and then sleeps from 7:00 pm - 7:00 am without any problems whatsoever. I'm pretty sure that he wakes up during his nap and during the night (like every normal human does), but because we did the hard work of teaching him how to go back to sleep without needing excessive pacifying he gets nice, solid blocks of healthy sleep. We don't have to shove a pacifier or boob in his mouth, rock him for hours on end, pat his back, or be present for him to fall asleep - we've taught him how to do it for himself and he's been this way since 6 months old! Sure, at this age he's awake in his crib and usually takes a little bit to fall asleep but he isn't crying; surprisingly enough he is saying good night to his Elmo and his 'sheepie' dolls and tucking them in with him.
You're totally doing the right thing - keep it up!