Yep.
iirc - Book of Vile Darkness and Book of Eternal Daisy's , both had a disclaimer that they worked for 3e and 3.5e.
Some 3e books had updates available on wizards.com, and some more 3e material also got updates.
Plenty of the 3e books had no update at all for 3.5e, but can be considered directly port-able into 3.5e
Alas, that is not truly true; there are minor interaction problems between 3e and 3.5e.
As long as the DM can wield rule zero with reasoned authority (not like Cartman), both editions get along quite well.
CO-board types cringe at the idea 3e + 3.5e , or also bringing in Dragon magazine. The interactions get worse.
But it is fun
.