A Comprehensive Grammar of the English Language by Quirk et al, in §15.74, says:
The adverb so premodifies an adjective or adverb, whereas the predeterminer such precedes a noun phrase. The adjective premodified by so may itself premodify the head of a noun phrase.
One of the examples they give is:
So few people came to the meeting that it was adjourned.
What is happening here is that "few" modifies "people", but "so" modifies only "few", not the entire noun phrase "few people". By way of contrast, in Gustavo's example:
There were such beautiful girls around he couldn't avoid gazing at them.
"such" modifies the entire noun phrase "beautiful girls".