I have no interest in rehashing Cholov Yisroel here, so please, just stick to this question.

If one drinks cholov stam, does that milk need a hechsher? Does it vary by locality?

Not as far as I'm concerned.

I believe so.

In Toronto, believe that restaurants that are not CY do not need milk with a hechsher but rather any store bought milk is ok.

To which question?

My father does hashgacha for a local dairy plant, and recently he only will buy milk with a hechsher. I haven't had a chance to ask if 1)it's a personal chumra 2)it's based on what he sees in the plant (not all the milk gets a hechsher) or 3)it's a widespread psak.

TBH, I have no idea, but know that my parents will only buy milk with a hechsher.

People buy water with a hechsher. That doesn't mean what they are doing is necessary or makes any sense.

Agreed which is why I said 'I think so' but gave no conclusive answer.

in europe, maybe. but why in america?

Not talking cholov akum, talking "cholov stam" -- the milk is processed, vitamins and hormones a lot of times are added (actually, I remember a specific announcement by the vaad hoeir in my hometown against a certain milk because the hormones in there were not kosher).

According to the list on Kosherquest.org, milk in the U.S. or Canada that has no ingredients other than added vitamins, does not need a hechsher.

I was told yes, because sometimes other stuff it put in the milk that can be derived from non kosher sources.
Like once in Boston they say that the milk was fortified with shark based vitamins. Now it has a heksher.....

Vitamin D3 [which may or not be a hormone] which is derived from, among other sources, sheep lanolin. And the emulsifiers to keep the Vitamin D3 in solution.
(Hormones are not added to milk. In Canada it's illegal, and in the US it's not done. But some consider D3 to be a hormone].

Whether D3 requires a hechsher depends whom one asks. If it does, then those certifying agencies that allow any milk in their products or to be commercially sold are being machtee et ha-rabim.

http://www.trauthdairy.com/02_mplusab.html
What about something like this? If it didn't have the hechsher, would it still be ok, even though it had things added?

I don't think it does, however I'm hard pressed to find one that doesn't have a hechsher. Especially since we only buy organic milk without rBST (not a problem in Canada where it is banned, as it should be) which tends to be brand name and have a hechsher.

I was surprised to read, I think it was from star-k, that cream and half and half DOES need a hechsher.

Cream requires a hechsher, because it is often blended with whey cream, which comes from cheese, and cheese needs to be gevinat yisrael according to everyone. (If you see it being skimmed directly from the milk, it doesn't require a hechsher, since it's just milk).

OU policy is that milk does not require a hechsher. In practice, if you use OU-D products you might as well use milk without a hechsher, since you are eating it anyways.

Star-k also issued an alert on "Kemps Plus Milks" because they have gelatin encapsulating the fish oil in the milk.
So probably any milk with any special ingredients one should be careful with...

London Beth Din policy is that plain milk and cream (also unflavoured yoghurt with nothing added) is fine. Of course, LBD says you can buy all sorts of things without a hechsher that normally come with one in the US...