On my Centos 6.4 Linux system (installed with default US settings) the collator is POSIX.
So...
[Zaphod@star5x2 ~]$ locale
LANG=en_US.utf8
LC_CTYPE="en_US.utf8"
LC_NUMERIC="en_US.utf8"
LC_TIME="en_US.utf8"
LC_COLLATE=POSIX
LC_MONETARY="en_US.utf8"
LC_MESSAGES="en_US.utf8"
LC_PAPER="en_US.utf8"
LC_NAME="en_US.utf8"
LC_ADDRESS="en_US.utf8"
LC_TELEPHONE="en_US.utf8"
LC_MEASUREMENT="en_US.utf8"
LC_IDENTIFICATION="en_US.utf8"
LC_ALL=
[Zaphod@star5x2 ~]$ echo $LC_COLLATE
POSIX
[Zaphod@star5x2 ~]$ cat f
b
a
B
A
[Zaphod@star5x2 ~]$ sort f
A
B
a
b
[Zaphod@star5x2 ~]$ export LC_COLLATE=en_US
[Zaphod@star5x2 ~]$ sort f
a
A
b
B
Cheers!
Z