This is only an partial answer, as I do not have the time now to look for other references.
In the chapter about milk, Harold McGee's beautiful book "On Food & Cooking" says (pg. 21 of the 2004 edition, bold is mine):
Flavors from cooking
Low-temperature pasteurization slightly modifies milk flavor by driving off some of the more delicate aromas, but stabilizes it by inactivating enzymes and bacteria, and adds slightly sulfury and green-leaf notes (dimethyl sulfide, hexanal). High-temperature pasteurization or brief cooking - heating milk above 170°F/76°C - generates traces of many flavorful substances, including those characteristic of vanilla, almonds and cultured butter, as well as hydrogen sulfide. Prolonged boiling encourages browning or Maillard reactions between lactose and milk proteins, and generates molecules that combine to give the flavor of butterscotch.
He continues by speaking about the different types of pasteurization, and says (pg.22-23)
The third method of pasteurizing milk is the ultra-high temperature (UHT) method which involves heating milk at 265-300°F/130-150°C either instantaneously or for 1 to 3 seconds, and produces milk that, if packaged under strictly sterile conditions can be stored for months without refrigeration. The longer UHT treatment imparts a cooked flavor and slightly brown color to milk; cream contains less lactose and protein, so its color and flavor are less affected.
Sterilized milk has been heated at 230-250°F/110-121°C for 8 to 30 minutes; it is even darker and stronger in flavor, and keeps indefinitely at room temperature.
So, it appears as if indeed there may be some Maillard reaction going on in UHT, although it is probably minor (otherwise the milk would be much darker in color, as it is the case for sterilized milk).
The book also speaks about homogenization, but it does not mention anything specific about Maillard reaction there (homogenization is not carried on at high temperatures, so it does makes sense not to have it).
To answer your question:
Does this effect the nutritional value of the milk?
Well, according to this presentation (but, be warned, it does not cite any reference!) it seems that it does, although I don't think the Maillard reaction is specially involved.
Specifically it says that the irreversible changes that occur are:
- Whey protein denaturation (35 to 100% of denaturation of $\beta$-lactoglobulin)
- Slight increase in the size of casein micelles
- Whey protein-casein micelle interaction
- Maillard reactions
- Fat globule composition changes (although no changes in nutritional value of milk fat)
- Vitamin content changes (20-30% loss in vitamin B1 and B12. Markedly reduced content in vitamin C and folic acid if high level of oxygen were present during the treatment, no changes in vitamins A, D, E and beta-carotene).
It also says that:
Biological value and net protein utilization of UHT milk were lower for stored milk than directly used milk. Lysine loss during UHT is high but not significant to affect milk nutritional values.
I don't know whether the fact that you have problems when drinking UHT milk has anything to do with Maillard reaction or any of the other modifications brought by UHT treatment, and I will not venture in there, as I am not an MD and prefer not to give any sort of medical advice.