Router: Linksys WRT3200ACM
Firmware: DD-WRT v3.0-r44251
If you need help to set it up, let me know.
There are later versions of firmware, and moving to a later (but stable) version has been on my todo list for a while. However, please note that EARLIER versions exhibited the same behaviour for WMM and OpenSprinkler, so newer versions are likely (but not certain) to require the same workaround.
The ESP8266/WMM interplay is also discussed here and there on the DD-WRT forums.