I think a big part of the problem is the increasing disrespect in our culture for labor rights and the concept of productive work. Look at meatpacking, for instance, which presently employs TONS of illegals, but only began to do so after the industry broke its unions by moving operations from traditional labor strongholds like Chicago to more "business-friendly" states. The inherent instability of an illegal workforce is way too attractive to the maximum-profit-at-any-cost types, as by their very status, illegals CAN'T complain or organize. And the realities of graft and patronage in Washington mean that enforcement against the companies hiring illegals will be tilted towards poor-schmuck middle managers, not the real decision-makers at the top...
We can argue about the morality and law of the situation all day, but the economic realities of supply and demand will ensure that the cycle moves on in its present fashion until some sort of check or balance is instituted. Jail time and felony enforcement will do a little, but I don't think it'll fundamentally change the situation. Only the re-development of a system of checks and balances between labor and management will accomplish that...