A 504 plan is actually part of the ADA (Americans with Disabilities Act) and is NOT part of IDEA (educational law). A 504 plan is basically just identifying that the child has a disability that justifies reasonable accommodations for success in the classroom. It is based on antidiscrimination laws, not equal access to education laws. 504 plans are often most useful for children with physical disabilities (ie: needs to have alternate seating in the classroom or needs to have someone help them get from class to class) but not cognitive disabilities. Schools sometimes prefer to do a 504 plan first when possible and appropriate - there's typically much less involved in terms of paperwork and needed resources for the child.
The current IDEA revisions have moved away from the discrepancy model and that's where RTI (Response to Intervention) comes in. The idea behind RTI is that the schools would be compelled to provide intervention to low achieving students without having to develop a full IEP (lots of time, paperwork, resources, etc). It was, I believe, a change in the act that was driven by NCLB - to help bring up the lower achieving students.
So, gifted kids *do* get left out of this. In order to get, say, a computer for typing a report under a 504 plan you would have to demonstrate that the child is being discriminated against, based on his disability, by NOT providing the computer. In order to get the computer under an IEP, the child has to be determined eligible for special education services (meet entry criteria for one of the eligibility categories, typically after full blown assessment by the team) and then the team must agree that the computer is an appropriate accommodation based on the presenting disability to help the child succeed *at an appropriate level* (read: grade level)in the classroom.
