I have been trying to figure out how to set my camera/flash to slave mode. Can I use my Canon 430EX II as an off camera flash with my Rebel T1i? If so, how would I accomplish this?
Unfortunately, your camera does not offer built-in wireless control of your flash. In order to control your flash off camera, you need to purchase the Canon ST-E2.
At this date, only the Canon 7D, 60D and 600D (Rebel T3i / Kiss X5) offer built-in wireless control of flash, but I assume that will change over time.
There are solutions from PocketWizard, and Radio Poppers that also support E-TTL remote, off camera flash support, but they tend to be a bit more expensive, but a great alternative if you have a challenging environment or shoot (around corners, long distance)
If you wish to remotely manually fire your flash (meaning not use E-TTL), you can get the 'Cactus' or 'eBay' triggers, that will allow remote firing of flash, but will not support the in-camera settings and control.