1. That's a good question, but I don't understand quantum field theory well enough to answer it. If I get some help, this answer will be updated.
2. Even in classical physics, negative energies are common. For example, the gravitational potential energy of two nearby objects is negative, if we start with the standard convention that when the objects are very far apart that energy is zero.
3. Oddly, it's the positive energy density of bosons which creates a repulsive effect in Einstein's General Relativity. (Einstein guessed correctly that some effect like that might exist, but then retracted the guess.) A negative background energy density creates an attractive effect. This is reversed from what you might expect essentially because the vacuum energy is not some fixed amount but rather a fixed density. As space expands or contracts the background energy associated with some particular region changes because the density is fixed and the volume changes. In contrast, most of the energy associated with simple matter is unaffected by the expansion of space, and the resulting gravitational effects are always attractive.
Mike W.
(published on 06/29/2008)