What prompted the Nesi'im to bring the Korbanos?
Rashi: After donating the wagons and the oxen to transport the Mishkan, they had an urge to donate Korbanos with which to inaugurate the Mizbe'ach. 1
What is the meaning of "Chanukas ha'Mizbe'ach"?
Rashi (in Lech L'cha, 14:14): It means the inauguration of the Mizbe'ach.
What are the implications of "Vayakrivu ha'Nesi'im Es Korbanam Lifnei ha'Mizbe'ach"?
Rashi: They brought their Korbanos in front of the Mishkan, but Moshe did not accept it, until Hashem instructed him to do so.
Ramban: Refer to 7:3:4:2**.
What is the difference between the first half of the Pasuk and the second half?
Seforno: The first half of the Pasuk is referring to declaring the animals Hekdesh; the second half, to bringing them in front of the Mizbe'ach.