This isn't really so much about any US allies not wanting it to happen as much as it is about the African Union not wanting it to happen. In January of 2008, the US State Department issued this statement:
"While the United States does not recognize Somaliland as an independent state, and we continue to believe that the question of Somaliland's independence should be resolved by the African Union, we continue regularly to engage with Somaliland as a regional administration."
This essentially states that the US is willing to concede the recognition of African countries to the African Union and is therefore following their lead. The primary reason that the African Union has given for not recognizing them is that they viewed Somalilands's claim as just another enclave seeking separation, and apparently this violates the principles of the African Union.
The African Union itself does not favour rearranging the borders of African countries, especially where there is no agreement. It feels that, rightly or wrongly, the colonial borders were fixed and that changing them would open up too much uncertainty.
The fact that Somalia refuses to recognize the existence of Somaliland is a significant factor in this. The matter is further complicated by the fact that there is another separationist movement in a tract of land called Puntland which joins these two regions. As long as this instability exists, it is unlikely that the African Union will recognize any country other than Somalia, so Somaliland and Puntland are both left out in the cold. Until the African Union decides to recognize these regions as separate countries, it is unlikely that any other major country will do so.
Somaliland's 'path to recognition'
BTW - The week of independence that you mentioned occured back in 1960. Since Somaliland has essentially been a part of Somalia for over fifty years, it is easier to understand why the African Union sees them as just another separationist movement.