The new Thunderbolt Display offers multiple USB ports, a FireWire 800 port, and a gigabit Ethernet port to computers hooked up to it all through one Thunderbolt cable, enabling, for example, a MacBook Air to not be constrained by lack of ports while at a desk. My question is can a 27" iMac (with Thunderbolt), while in Target Display Mode, offer the same ports via thunderbolt cable? Aka: if I get a new MacBook Air, do I get all the ports through my 27" iMac as I would through a Thunderbolt Display?
No. Devices connected to the iMac's other ports will still only be accessible by the iMac, not by the connected MacBook.
You can approximate some of the functionality you might get from this by using various software solutions to share your iMac's devices - such as sharing drives over the network, or sharing the mouse and keyboard using Synergy, etc.