I am one who largely agrees with you about the need for immigration reform, however, Bush bought non-enforcement when he said that we needed guest workers to do "jobs Americans won't do". Another big stumbling block for any immigration legislation Bush endorses is that Chertoff has been making statements that are easily interpreted as, we will only do enforcement if we get amnesty.
Bush also lost a lot of good will in the debates about immigration reform last year when he seemed to consistently come out in support of the bill with the weakest enforcement provisions and the widest amnesty.
The problem with this is that once there is amnesty, there won't be need for enforcement because there won't be anything to enforce.