Part of the answer can be found here. Judicial Watch used the FOIA to request a survey that the Border Patrol did of detained illegal aliens.

The key findings from the survey:

1. President Bush's proposal lured greater numbers of illegal immigrants to violate the law. Approximately 45% of respondents crossed illegally based upon rumors of a Bush amnesty program.

2. Approximately 63% of the illegal aliens surveyed received Mexican government or media information supporting the notion of a Bush administration amnesty program.

3. When asked if they would seek amnesty if offered, 80% of apprehended illegal immigrants answered, "yes."

JW also obtained a memo entitled "White House Approved Talking Points":

[It] instructs Border Patrol agents to withhold information about the negative impact of the president's proposal. "Do not talk about amnesty, increase in apprehensions, or give comparisons of past immigration reform proposals... Do not provide statistics on apprehension spikes or past amnesty data..."

UPDATE: Additional reports in:

KRT/DMN: Administration accused of withholding information on immigration

CNS: Bush Encouraged Illegal Aliens, Congressman Says

AP: Lawmaker wants probe on immigrant survey

Human Events: Survey Analysis Indicates 'Guest Worker' Proposal Lured More Illegal Immigrants

Press-Enterprise: White House accused of hiding data

WaPo: Bush Proposal Prompted Surge in Illegal Immigrants

AZ Republic: Group rips Bush gag on border surveys