Grepping through kernel configurations of Debian:
linux-image-2.6.26-2-486_2.6.26-15lenny3_i386.deb CONFIG_BLK_DEV_LOOP=m
linux-image-2.6.26-2-486_2.6.26-15_i386.deb CONFIG_BLK_DEV_LOOP=m
linux-image-2.6.30-1-486_2.6.30-1_i386.deb CONFIG_BLK_DEV_LOOP=m
Same for RHEL4 and RHEL5.
The only one that I found, which made the same choice of not building loop as a module is Fedora 11: kernel-2.6.29.4-167.fc11.i586.rpm CONFIG_BLK_DEV_LOOP=y
So I think Gentoo is not the only one making this possible.
Grepping through kernel configurations of Debian:
linux-image- 2.6.26- 2-486_2. 6.26-15lenny3_ i386.deb BLK_DEV_ LOOP=m
CONFIG_
linux-image- 2.6.26- 2-486_2. 6.26-15_ i386.deb BLK_DEV_ LOOP=m
CONFIG_
linux-image- 2.6.30- 1-486_2. 6.30-1_ i386.deb BLK_DEV_ LOOP=m
CONFIG_
Same for RHEL4 and RHEL5.
The only one that I found, which made the same choice of not building loop 2.6.29. 4-167.fc11. i586.rpm BLK_DEV_ LOOP=y
as a module is Fedora 11: kernel-
CONFIG_
So I think Gentoo is not the only one making this possible.