Poplar would not be my first choice for a stain grade wood. Oak or Cherry looks much nicer. Poplar is usually used as a paint grade wood.

actually Poplar when stained looks very much like cherry for a fraction of the price. I would try and select the boards without a lot of the green colored grain to it. I would also recommend using a wood conditioner prior to staining as well.

Thanks everyone. This stained poplar that we are looking at does look very much like cherry. A little lighter shade. But any advice on how it would look as trim for a white window?

Here is a link that might be useful: poplar

i think it would look fine, I have seen alot of houses with stained trim and white windows

Why not just go with Cherry if that's what you want it to look like?