Saying "Ikimashou" is a very direct way of saying "lets go". In Japan, it's considered rude to say this to someone, especially if you don't know them very well. "Ikimasenka" is a negative, but it can be interpreted as "You want to go, don't you?", although it's usually thought of as "Do you want to go?". It's normally used in a sentance, rather than singularly.
eg:
レストランは行きませんか?
(レストランはいきませんか?)
RESUTORAN wa ikimasenka?
Do you want to go to the Restaurant?
Using "ikimashouka" probably has the same effect, and I think it can be used in less formal situations. Hmm, I dunno.
Anyways, gomenasai minna-san! I haven't posted a lesson for AGES!!

I'm so busy these days, but I'll try and get one together for everyone really soon.
I've got an Oral today on "Watashi no Kazoku" (My Family), which is worth 15% of my mark. I hope I don't forget what I'm supposed to be saying... I hate public speaking.