The major medical reason that we've chosen to delay vaxes is that the blood/brain barrier is not established in infants. The vax is injected into the blood stream, bypassing many initial immune system defenses. The immune system is not ready to respond to the assault of all of the diseases, plus the ingredients in the vaccines. For more information on this line of thought, read the book What Your Doctor May Not Tell You About Children's Vaccines, by Dr. Stephanie Cave.
This is my favorite vaccine article: A User-Friendly Vaccination Schedule. I would call this research that supports delaying vaxes. http://www.lewrockwell.com/miller/miller15.html
Hep B: Is primarily transmitted through sex and needle-sharing. The only reason infants get it is that they are a captive audience. By vaxing infants, the thinking is, we'll prevent them from getting/spreading the disease later on. However, there are correlations between the Hep B vax and SIDS. Babies don't need it.
The ingredients in the vaccines also give me pause. Many vaxes are made using cells from other animals or from aborted fetal tissue (I'm pro-choice, but I don't think that aborted fetal tissue needs to be injected into otherwise healthy babies). http://www.informedchoice.info/cocktail.html
Finally, the fact that there are 33 doses of vaccines given before age 5. A 2 month old gets 7 vaccines. How can an undeveloped immune system handle Diptheria, pertussis, tetanus, meningitis, polio, pneumonia, and Hep B? PLUS: formaldehyde, aluminum hydroxide, aluminum phosphate, thimerosal, polysorbate 80, ammonium sulfate, formalin, sucrose, neomycin, streptomycin, polymyxin B, 2-phenoxyethenol, monkey-kidney cells, phenol, and probably some other undesirables.
So, that's my rationale.