[Update: I wound up writing a more detailed analysis of the new data plans and their implications, for posting as a blog entry for my iPhone book. You can read it here.]

AT&T today announced its new data plans for iPhones and iPads (as well as for all their other smartphones). A Mac Observer article summarizes the changes, with full details listed on an AT&T Web page.

The new rules are good news overall — as they will save money for most iPhone and iPad users. But there is one big exception: iPhone tethering.

Briefly, there are 2 plans to choose from: $15/month for 200MB of data (DataPlus) or $25/month for 2GB of data (DataPro). The former $30/month “unlimited” options are completely gone (for the iPad, “unlimited” vanishes only a month after it arrived!).

If you exceed the caps, you pay additional fees. For example, with the 2GB cap, you pay $10 for each extra GB. The new plans go into effect June 7 (although you can retain your current contract if you wish).

Basic plans likely to save money. While the loss of any “unlimited” option is disappointing, most iPhone users should find that they save money with the new plans. As a personal example, I use less than 2 GB of data per month on my iPhone while my wife uses less than 200MB. This means that our combined new monthly data costs for our two iPhones will be $25 + $15 = $40/month. This is a $20 saving compared to the $60/month we are now paying — and for no change in our usage patterns. Great!

For iPad owners, the $15/month contract gets you 50MB less data than the current $15 iPad plan. [Correction: iPad users will remain at 250MB for $15.] Otherwise, the changes amount to the same as for the iPhone. However, iPad users are more likely to reach a 2GB cap than iPhone users — because they’re more likely to do video streaming. Still, I suspect that most iPad users do the majority of their video watching over Wi-Fi, where these limits and fees do not apply. So, even with the iPad, I expect that most owners will wind up spending less per month than under the old plans.

Tethering plan discourages its use. The one big fly in the ointment is iPhone tethering. Apple added tethering as an option last year, when iPhone OS 3.0 was released. But AT&T never enabled the feature. They will finally do so when iPhone OS 4.0 comes out later this summer. My initial reaction to this news was to cheer: “At last!”

However, the pricing structure for iPhone tethering quickly dampened my enthusiasm. Turning on tethering costs an additional $20/month. Further, the option is only available for DataPro users. In other words, having tethering available requires that you pay $25 + $20 = $45/month.

If you have the DataPlus plan and a situation crops up where tethering would be useful — and you only need it for a few hours — you’ll need to spend an extra $30 for those few hours. I would have much preferred a reasonable pay-as-you-go hourly option — available to both DataPlus and DataPro users.

As an added irritation, iPhone tethering usage counts against your 2GB cap. This amounts to double-charging. That is, the $20/month tethering fee gets you nothing except permission to use tethering; there is no data allowance included with the fee.

As it now stands, unless you know you will be using tethering often, AT&T’s plan discourages accessing it — which is probably deliberate. AT&T stalled on even enabling tethering; this is their reluctant concession. I foresee their policy driving more people to consider jailbreaking their iPhones — and using tethering apps such as MiWi.

Question. There is one final point that I am still unsure about: Can users turn on and off the new iPhone plans on a monthly basis, the way you can do with iPads? Or must iPhone users always have at least the DataPlus plan active? We are waiting to hear back from AT&T on this. [Update: It appears that iPhone users will be required to use at least the DataPlus plan.]