This should be possible. For exemplar, the OpenSprinkler can be connected to Pi with a cross-over cable, and then Pi is connected to the main router through USB WiFi.
Just to give everyone a heads up: I am working on an OpenSprinkler Pi edition: basically it’s a shield that’s wired to Pi through the GPIO pins, so the Pi can directly control the shift register and hence the solenoids. The board is essentially a subset of the current OpenSprinkler hardware, with 24VAC->5V power conversion, shift register and triacs, and DS1307 RTC. The 24VAC->5V is capable of outputting 1A current, so it can power a PI board together with a WiFi USB dongle. The hardware has been tested and is working fine. It’s pretty straightforward and the nice thing is that I designed some copper pillars to support the Pi board on top of the shield, so that everything can fit inside the existing OpenSprinkler enclosure. The software is not finished yet: I’ve written a piece of Python code that runs a very simple http server, presenting a list of buttons and allows you to control solenoids manually (very much like the first manual control program for OpenSprinkler which I wrote a long time ago). I am hoping to post about it some time this week and next week. This should probably attract some interests from people who have a Pi at hand.
Some pictures for the curious: