First of all, it's "ittekimasu", in hiragana いってきます and it's not rude at all. We use it in our everyday conversation. It's one of the greetings and it's used by the people who are leaving their house to their family members who are at home. We also use it before trip to family, friends, etc who stay their home, hometown, country, etc. I hope it helps you.

And to add to Ryoko-san's answer, "itte kimasu" means "I'm going (iku, -te form) and I'm coming back (kuru)". So it's less like "I'm leaving now" and more like "See you later (when I'm back)". Not rude at all. :-) The usual answer to "ittekimasu" is "itterashai", and the fact that there exists a formalized answer also makes it very clear that this is a polite exchange.