The wiki says "Luckily, starting from 2.6.34, the kernel has IGMP snooping feature for the software bridges (disabled by default in OpenWrt) which should prevent unnecessary traffic on ports that were not actually subscribing" So it depends on the kernel you have. I would assume this feature is available in kernel 2.6.34 and newer.
I mean the wiki at
http://wiki.openwrt.org/doc/howto/udp_multicast
Anyway, my HG556A Barrier Breaker router is running
root@Vodafone-HG556A:~# cat /proc/version
Linux version 3.7.9 (openwrt@openwrt) (gcc version 4.6.4 20121210 (prerelease) (Linaro GCC 4.6-2012.12) ) #10 Su n Mar 17 12:16:25 EST 2013
Whereas my rt305x router is running
(Attitude Adjustment, r35752)
root@rt305x:~# cat /proc/version
Linux version 3.3.8 (openwrt@openwrt) (gcc version 4.6.3 20120201 (prerelease) (Linaro GCC 4.6-2012.02) ) #3 Sun Mar 31 00:32:18 EST 2013
Had a quick look. OpenWRT 10.03.1 has kernel 2.6.30, so it would be in later release than last official "stable". Latest official beta (Attitude Adjustment 12.09) should be ok since is has a 3.x kernel. Trunk (nightly builds) are obviously ok too.
Maybe you missed out something since I don't see this in my 3.x Openwrt ROM running Attitude Adjustment or Barrier Breaker.