1. "social security for illegals who have worked for 18 months with someone else's SSN" isn't law, and isn't in effect.
2. What the alleged agreement ("awaiting Bush's signature" (since 1994!) really means, or if it even exists, will await further research, IMHO. Usually, when a group breathlessly announces that they have uncovered something that was been secretly agreed to years ago by mysterious forces, there is a lot less there than meets the eye.
But, I am willing to research further.
http://waysandmeans.house.gov/media/pdf/ss/qandamexico.pdf
It confirms that there is an agreement that MAY be submitted to Congress -- but that is not in effect. It refutes a lot of the rhetoric above. It's not "Full" ; It's not for illegals; It is reciprocal.
Maybe it's a good idea; maybe it's not. Maybe the committe is right; maybe it's not. But let's argue based on the facts as they come out, not casually toss out that it is in effect, with the most exaggerated interpretations.
That said, one of the interesting things in the Q & A is that the "one-house veto" provision to which the agreement would be subject has been struck down by the Supreme Court in other contexts (INS v. Chadha), Whether that means that the whole process would be unconstitutional without affirmative legislation would seem to be an open question.
It will not help your pocket book to stick your head in the sand. Just search illegal+Social Security and read. This bill was passed in May of 06 BTW.