Bob's answer about the intended meaning of 'not yet going' is on the right track but it leaves out a couple of important details.
There are more than one set of Greek manuscripts and there are actually quite a number of one and two character type differences between them. Usually, these involve punctuation or spelling differences and have no affect on the text; but sometimes a snippet can be a little bit different and it's unclear which set of manuscripts to go by.
It is important to note that NONE of these differences in the manuscripts used to produce our translations touch on ANY doctrinal issue so there is no need to fret about that. Even when a word is dropped, the meaning can be matched with the same doctrine in other places in the text. However, sometimes odd linguistic issues like the one in this question do come up. There are at least a dozen or so that I've read about and probably more that I haven't.
First of all, the real trouble is not actually in verse 6 which was [originally] quoted by the OP. In verse 6 Jesus is talking his final hour and time to go to the cross. That is is what he means when he said his time has not fully come, but the disciples completely misunderstand this. Not just this time either: he often references his death and they don't really get the picture. In verse 6 there is no particular problem with the translation, 'not yet' is appropriate, but the subject matter is not understood.
The real issue comes in verse 8 where Jesus actually IS talking about physically walking to the feast. This is the verse that Bob quotes and contains the word issue. In quite a number of the older manuscripts, the word in question is 'oupō' as quoted and is correctly translated 'not yet', but this word is found in other manuscripts as 'ouk' meaning simply 'not'.
So which is it? The answer is it doesn't matter.
In English, the use of present continuous tense "I am not going" can and often does apply to a events in the future. That would mean the same thing as "I am not going to go at all" However the verb tense used in Greek here simply meant "I am not now going" ... as in he wasn't going to budge that moment and go like they wanted. It wasn't an indication that he didn't maybe plan on going.
Thus, whichever set of manuscripts you choose to go by, whether you use 'not' or 'not yet' in verse 8 is a matter of discretion of the translators. It is not necessarily a mistake in the NIV or any other translation to use simply 'not' because the best manuscripts have it written that way in Greek, however because of the verb tense we also know that whether the alternate 'not yet' Greek version is original or not, that sense is indeed intended by the original author of the text. You will find many translations that use 'not yet' because it gives us a clue about what is being said that is lost in our English verb tense.
And no -- any way you add it up, Jesus was not lying. He did not jump up and do what the disciples wanted him to do (verse 8) nor had his final time come (verse 6).
