The subject says it all, when playing Domination, do captures of a base count toward your point streaks? Seems like it id for me in one game; I got a support pointstreak award after capturing, but wasn't sure if that was related to something else (like the grenade I threw)...
Yes, capturing a point in domination counts the same as one "kill" for the purposes of pointstreaks. I don't know all the breakdowns for the various game modes, but I do know that capturing the flag in the relevant game mode counts the same as well.
You can watch your pointstreak counter in the lower right to see how much progress you're making towards your next streak. This can help you understand what counts and what doesn't.