In general, decaf tea should give you the same types of benefits as regular, but the strengths of those benefits will be lessened. The actual degree to which they're lessened is up for debate, though.
Studies on the precise strength of the health benefits of tea are generally very mixed, due to the huge variability of tea types, sources, and qualities, not to mention the many different brewing factors (time, temp, etc), storage and packaging conditions, and methods and types of measurement. Decaffeination almost certainly will reduce the health benefits by some amount, but that amount might be dwarfed by some other factor.
In short, decaf is probably not as healthful as regular, but it's still good for you. =)
(Wang, et al 1994) did lab tests on mice and concluded that black tea, green tea, decaffeinated black tea, and decaffeinated green tea were all effective at inhibiting skin cancer, but that the decaffeinated brews were "slightly less effective".
This recent story, by a writer for the American Institute for Cancer Research, claims that decaf green tea has roughly 1/3 the antioxidant levels of regular green tea (for the most-touted antioxidant, EGCG.)
The USDA Database of Flavonoid content of food has a huge table which shows the levels of different flavonoids for different types of tea. Decaf is almost always below regular, although there's no consistent ratio.
Another article claims decaffeination reduces the catechins in black tea by a factor of 14 (but only by a factor of 5 for green tea.)