Sorry about the confusion — those instructions (“Note that OpenSprinkler does not use a bootloader, so please do not upload any bootloader to it”) are outdated. It was for OpenSprinkler 2.0 which doesn’t use bootloader. All later versions (2.1, 2.2, 2.3) use a bootloader. Still it’s good to be cautious about the bootloader because you should be explicitly aware of this bootloader you are uploading.
Note that OpenSprinkler 2.2 uses optiboot bootloader (only OS 2.1 uses USBasp). The optiboot bootloader for ATmega644 can be found at:
http://code.google.com/p/optiboot/downloads/list