basically MEXT will contact both the university admission office and the professor. normally, by this time.. MEXT will already finish their discussion with most universities, and with both agreeing on accepting the student. however, the FINAL decision is not from the university or the professor, but by MEXT. so, there are chances that the university had agreed with MEXT, thus telling the student that you are sure to come, and started discussing on accommodation and what to do after arrival, only to know later that MEXT comes up with an official letter that you are not going after all. so, WAIT for the MEXT confirmation then we shall celebrate!
I think the "tentative letter" means MEXT has approved our scholarship. Refer to the heading "Result of Nomination" (our embassies nominates us to MEXT, hence MEXT reviews, makes a decision and inform our embassies on the results of nomination) So university is the last step. If university says yes, then it should be fine.
If you read last year's posts - towards the end of December, many got calls from embassies telling them that they have been chosen, and they need to wait till Feb to know their university placements. This year, for Malaysia, we got the letter instead of phone calls, and it's very early - 17 Dec. Hence we got a bit confused. Supposedly that IS the results. We have been chosen. (By refering to last year's situation). Perhaps the word "tentative" is really disturbing, and the tone of the letter doesn't make us excited.
Perhaps it's the way the message is conveyed:
1. You are selected, kindly wait till Feb for your university placement.
2. You are selected, but SUBJECTED to successful university placement, if there are no university that may accept you, we have the right to CANCEL the scholarship.
Hence, by receiving it the 2nd way, we tend to be more cautious.
So for those who has got the tentative letters, if your university contacted you for hostel arrangements, GO FOR IT, and hurry. It's far cheaper than private residences.
But what kimberly said is true - Nothing is official yet. The last cue is university. So if they contacted you, you're in.