I think they got their pick up really early, so I'm pretty sure they knew, even if it wasn't officially announced. ITA though, it would have been a good (if not a bit unsatisfying) series finale.
I just looked it up and they got renewed for season 3 in January. That's a pretty big vote of confidence!
I don't want Castle and Beckett to get together too quickly, but I don't want it to NEVER happen. Frankly, when shows drag it out longer than is reasonable, I tend to lose interest. I guess I don't like being teased that much.
Ideally, they'll get together next season and, by the end of the season they'll have screwed it up. Then, when the series ends down the line, they can get them back together for good.