One of the big mysteries of Inland Empire is the tatoo of "L.B." on Sue's hand during the monolog scenes.

Could mean a thousand things...

But first of all, 1., Lynch's newest born daughter (with Emily Stofle), they named her Lula Boginia.

2., and more obvious so I'm sure it's been mentioned before.

James Stewart plays "L.B." Jeffries in Rear Window.

Only connection to Mulholland Drive I can think of is.......... Louise Bonner.

The daughter must have been conceived/born after IE, so I don't think that's it.

As for Rear Window, I can't really see why this would be referenced in this context.

Lost Boy is a possibility, but I remain unconvinced. In fact, I've never heard a really convincing suggestion.