David G. Cassidy
Every NIH institute has its own IT operations. Some of these are larger than others — NCI, for example. CIT provides centralized IT services for all of NIH, and offers hosting and other cross-agency services to the institutes.
NITAAC is an NIH acquisitions operation that services agencies across the government, not just NIH. They run contract vehicles such as CIO-SP2i (soon to be CIO-SP3), ECS, and ImageWorld2.
If you want to get involved in IT at the NIH, you should think about what kind of IT you want to be doing, and then address your queries to those institutes (or CIT) as appropriate. So if you are interested in bioinformatics, for example, you might talk to the folks dealing in that areas in NCI, NIBIB, NCRR, and so on.
Hope this helps!