In Knowledge Base document #306412, Apple reveals that iTunes WiFi Store downloads can be paused, interrupted and resumed -- even if an iPod touch or iPhone is turned off in the middle of the download process. Additionally, if you access your iTunes account on a desktop computer after initiating a download on the iPod touch or iPhone, it will complete the download process and sync the files to your device.
The document states:
"If you need to turn off (your iPod touch or iPhone) or leave the area of your Wi-Fi connection, don't worry about interrupting the download. (The device) starts the download again the next time (it) joins a Wi-Fi network with an Internet connection. Or if you open iTunes on your computer, iTunes completes the download to your iTunes library."