i would use a real charger IC, nim-h get really screwed up without current limit and overcharge limit.
as long as the charger doesn't pull more than 500mA (the usb limit i think) any one should be fine. switchers are better than ldo, but it's up to you. try the maxim or linear tech sites
I have read an article regarding USB universla phone charger in the EDN online magazine. It uses an IC produced by Linear Technologies. I forgot the part number, it starts with LT?????. Connecting directly to the USB port may damage your phone or the port especially if it draws more than the said capacity.