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Adverbs and adverbial phrases can move pretty freely in English. These all sound fine to me:

we are going to go to the airport by bus tomorrow

we are going to go by bus to the airport tomorrow

tomorrow, we are going to go to the airport by bus

we are going to go to the airport tomorrow by bus

we are going to go tomorrow to the airport by bus

The last sentence sounds a little more awkward, but if you're emphasizing when the event is occurring, it would be fine. Similarly, in the right context:

to the airport, we are going to go by bus tomorrow

by bus, we are going to go to the airport tomorrow

This phenomenon is called topicalization.

http://en.wikipedia.org/wiki/Topicalization