When Snape tells everyone to practice by attempting to jinx one another (and repel the jinxes) he doesn't mention any specific spell or any restrictions on spells not being able to be used non verbally. The only thing he mentions is that
Not all wizards can do this, of course; it is a question of
concentration and mind power
Hermione also mentions that they are useful in that your opponent won't know what kind of magic you are using:
"Your adversary has no warning about what kind of magic you're about to
perform," said Hermione, "which gives you a split-second advantage."
Canon seems to support that there is no restriction on what kind of magic you use. You seem to be only restricted by your own concentration and mind power. However, I could find no evidence of one of the unforgivable curses being done non-verbally, and as they require immense concentration in and of themselves, it is unlikely that any but the most skilled wizards would be able to perform them silently. (Voldemort is a likely candidate for possibly being able to however, he seems to revel in the fear instilled in a victim just by hearing the curse be spoken to them.)
The closest evidence I could find contrary to this belief was during Dumbledore's arrest attempt:
As Dumbledore spoke, Harry heard a rustle behind him and rather thought Kingsley whispered something.
"Unfortunately, I had to hex Kingsley too, or it would have looked very suspicious," said Dumbledore in a low voice. "He was remarkably quick on the uptake, modifying Miss Edgecombe's memory like that while everyone was looking the other way-..."
This particular situation would have been the perfect time for Kingsley to use a non-verbal spell, as if he were caught modifying her memory it would have been potentially disastrous. However, this is before non-verbality in spells really comes to fruition, and I suspect the artistic merits of Harry hearing a whisper would supersede a non-verbal spell in any case. (I should point out of course that not using a non-verbal spell is in no way evidence that it can't be used).
In conclusion, there isn't a time in canon when an Unforgivable Curse is explicitly stated to have been used non-verbally, but based on the canon evidence of non verbal spells it seems that they could potentially be used that way.