Folks like to say never, but as others say, you should never say never... BUT... the reason they say never about this subject is as follows:
Apple patched the exploit used by ion1c to untether the 4.x jailbreaks in 4.3.4 and 4.2.9.
It is a lot of hard work and effort to discover a way to untether a jailbreak. With IOS 5.0 just around the corner the question is why would a talented jailbreak developer waste any of their time searching for, and assuming they find one, then exploit a bug to make an untether for 4 versions of IOS (4.3.4, 4.3.5, 4.2.9, 4.2.10) which are already more of less obsolete?
Doing this stuff is not trivial. It takes many weeks / months of hard slog. That is why most folks say it will never happen. Nobody who has the skill is motivated to do it. They are spending their time finding exploits for IOS5 (and as we know from recent news they have 5 userland exploits already in their bag).
You have the option of a tethered jailbreak in the meantime. You just have to live with that. If you are careful about how you used your phone you can live with the tether, provided you take the right precautions.
Also, if you want to be a little more optimistic, one of the userland bugs discovered for IOS5 may actually one day allow the 4.x versions to be untethered too. But there is no way that will be released until 5.x is well and truly out. Then and only then you might see an untether for them, but that is a really long shot and by the time it comes out you will probably rather want to be on the 5.x jailbreak it is associated with...