I personally have a slight beef with the term "voiding the warranty" as a knee jerk reaction to jailbreaking.
The warranty very specifically covers the hardware and not the software one bit.
The software isn't covered at all in the warranty. The device is legally not fit for any purpose, may not work, may not work as intended and may not be suitable for a whole range of lawyerly things.
Consumers don't own the software - just a temporary and limited license to use it under very specific circumstances. Modifying the software does break that license and although companies can take action, they usually encourage you to revert the changes or allow them to wipe the device and attempt to set it up correctly before spending any more time troubleshooting a software issue. It's rare to get hauled to court over breaking one or even ten license agreements but you're further out on a limb than if you don't jailbreak in the first place.
So the technical and social disadvantages of a jailbreak include:
- Reduced software support from Apple in case you need help
- Potentially circumventing security measures and opening your device up to compromise - you need to understand how to mitigate your jailbreak and the source of the code you run since it could be a trojan and do things you don't want
- Potentially less stable or totally failing software - especially any time the device is updated or requires activation after a restore.
- Inability to run some apps from the app store
- Inability to get more functionality from in app purchases
- Inability to rely on Apple's notification services
- Potential to have hardware claims denied
- Automated backup solutions through iTunes are harder and may require you to learn how to back up your phone with other tools
In the case where software could impact the normal operation of the hardware, you are at the mercy of the company if you bring them a phone that's been modified in a way that you were asked to not modify. Specifically, a jailbreak could fail and cause the boot portion of the phone to become corrupted. If you break a lock that you yourself have been trying to pick. The company could easily take the stance that your picking broke their otherwise fine mechanism. They might also cover the repair if they truly feel your actions were irrelevant to the specific warranty claim you are making.
As to future updates undoing the jailbreak - that has been the case for many updates. It's folly to predict what may happen in the future since things could get more restrictive, less restrictive or stay the same where you do have to unlock again and again as you update.