Nullriver says that Apple pulled it again, without explanation. I suppose it’s possible that it’ll mysteriously return again, but at this point I think it’s more likely that it’ll remain an application non grata until and unless AT&T formalizes tethering as a legitimate iPhone use. Which, as I keep saying, I hope they’ll do–and it doesn’t seem unthinkable that they will, since other AT&T phones get that option.

Meanwhile, judging from the copy of NetShare that I managed to buy during one of its windows of availability, I’m not heartbroken that this particular iPhone tethering app keeps coming and going…

Today? NetShare is back in the App Store. I’ve seen no official word from Apple, AT&T, or Nullriver about what’s going on here, and whether using NetShare is okay, not okay, or in some fuzzy area in between. I’ve pined for the ability to tether my notebook to my phone’s modem ever since I dumped my AT&T Tilt phone for an iPhone 3G, though, so I downloaded, configured, and tried NetShare as fast as I could.

I’ve given it only a quick try so far, not an extensive trial. For a few minutes I was up and running, using Safari to surf the Web (including YouTube) at decent speed and chatting on iChat. That was a kick. But other than that, NetShare has been kind of a pain–it’s safe to say it’s not the seamless iPhone tethering solution I’ve been looking for and would happily pay for.

NetShare works by setting up a peer-to-peer Wi-Fi network between your Mac (Windows support is promised soon) and the iPhone, then using proxy access to let your Mac piggyback on the iPhone’s internet connection. This requires some futzing around with network settings on both the iPhone and the Mac; Nullriver provides instructions, but they’re confusing in some places and simply wrong in others, and they’re far from complete. They also don’t mention the fact that they only get you as far as making Safari and iChat work; I had to reconfigure Flock to use NetShare’s proxy myself.

(Side note: A forum member over at MacRumors has provided instructions for using NetShare that are much better than Nullriver’s.)

Every time my iPhone went into sleep mode, my network connection died. I reconnected only by randomly fiddling with network settings and never figured out what I did that would get the connection working again. And when I was done testing and wanted to get back on my Wi-Fi network, I had to undo some of the settings that NetShare required me to change.

In short, if you’re a geek with a need for a way to connect a Mac via iPhone for emergency use, NetShare might fit the bill. But I found it way too much of a hassle to contemplate using it very often, even if AT&T were to give NetShare its blessings.

The good news: NetShare proves that there’s no technical reason why an iPhone can’t double as a modem, and the fact that it’s on the App Store would seem to suggest that it’s not unthinkable that a more polished tethering application along the lines of June Fabrics’ PdaNet couldn’t be approved by Apple and at least grudgingly accepted by AT&T.

But I say it again: If a future upgrade to the iPhone OS had some sort of official “Tether to Computer” option, as my Tilt did, I’d cheerfully fork over money to AT&T.

And I’d love to learn exactly what the deal is with NetShare’s appearance, disappearance, and reappearance, and whether Apple is really selling a piece of software that AT&T doesn’t want you to run…