It's looking unlikely you will ever be able to take a JTAG onto Xbox Live once Kinect arrives, not even for 5 minutes. The JTAG's have a rebooter which needs to be updated with components from the lastest dash. It looks unlikely there will be an updated rebooter, the current rebooter FreeBoot is abandoned and the sourcecode hasn't been released. Given some people have migrated to the PS3 since it got jailbroken, it doesn't look good for someone else to come up with a rebooter. JTAG's are really for use offline, or over system link.
For offline gaming this is less of an issue, you can strip out the $systemupdate that is included on game discs. The next version of ISO2GOD will have that as a built in option, and ISO Extract 0.6 already has it too. You can also recreate an ISO from the files ISO Extract produces, and then use ISO2GOD to install as a Game On Demand.
Even before this dashboard you couldn't realistically take them online without them being banned in a matter of hours. Because JTAG's can run unsigned code, people have used them with modded games. They then host modified lobbies allowing retailers consoles to join and hack their profiles etc, people also use them with aimbots etc so they can cheat at CODMW2 and so on. It's a waste of a JTAG.