Is water still infinite with BuildCraft?

Depending on the pump you use. The Redpower pumps suck up water blocks, eventually draining a water source entirely, but IndustrialCraft pumps are not so "efficient". They will only empty a single water block, so as long as you have an infinite water source, they will pump the fluid indefinitely. A good way to think about it is an IndustrialCraft pump will function the same way as a bucket - if you have an infinite water source, the pump will continue without issue.

Thermal Expansion have an alternate option, called an Aqueous Accumulator. It's a little expensive to build, but this is the machine you are looking for. All you require is a small infinite water source and the Accumulator will draw from this indefinitely.

Here is the recipe for the Accumulator.