Just to counteract the Blame Bush First mentality-
The trend you posted is obvious, but the reason many attribute to it (President Bush) is incorrect. A lot of people are falling into the old statistical fallacy that correlation equals coordination (ie, Bush was elected in 2000 = drop in immigration enforcement). This is a fallacy of both logic and statistics.
What did happen during this time period that directly effected immigration enforcement? Let's look...
Clinton signed the following laws into place.
(INA § 203(b)(2)(B), 8 U.S.C. § 1153(b)(2)(B)) - This law allows foreign nationals- even those who entered illegally, to request/obtain green cards as soon as they are 'caught'. What does this mean? It means that if caught and if they haven't committed a felony (remember, entering the country illegally is only a misdemeanor), they can simply say they would like to apply for a 'green card'-
This law was originally intended to help 'highly qualified' foreign nationals who want to get work permits easier, but the loophole is that the alien (legal or illegal) must be given the opportunity to prove they meet the qualification.
No, most illegals don't qualify for this visa waiver, but, by the time this is 'proven' they have fallen off the radar.
Worst of all is the The Illegal Immigration Reform and Immigrant Responsibility Act of 1996 (IIRAIRA). Under this law, deportation can be suspended if the illegal alien can prove that they have been in the US 'for a considerable period of time' and has 'good moral character'. Illegal aliens who fall under this are given the opportunity to apply for whatever work visa or asylum they may work for.
Is this an excuse to why something hasn't been done legislatively- No? this is simply trying to clear up the real causation of this statistical drop in enforcement.
But what about employers, why aren't we acting against them.. well, thank Clintoon again...
A Clinton era law (INA 274B) made it illegal for employers to ask an employee if the information they provided is 'valid', instead, as long as the employer has an I9, valid information or not, they cannot be prosecuted because they are not responsible for validating the information on the I9.
Basically, if an employer questions an employee 'if they are legal' or if they are providing false ID, they could be sued for 'unfair employment practices'
Sec. 274B. [8 U.S.C. 1324b] (1996)
(a) Prohibition of Discrimination Based on National Origin or Citizenship Status.-
(1) General rule.-It is an unfair immigration-related employment practice for a person or other entity to discriminate against any individual (other than an unauthorized alien, as defined in section 274A(h)(3)) with respect to the hiring, or recruitment or referral for a fee, of the individual for employment or the discharging of the individual from employment-
(A) because of such individual's national origin, or
(B) in the case of a protected individual (as defined in paragraph (3)), because of such individual's citizenship status.
http://www.uscis.gov/lpBin/lpext.dll/inserts/slb/slb-1/slb-20/slb-8468?f=templates&fn=document-frame.htm
It is good to note and complain about the downward trend, but don't fall for bad causation assumptions.