EDIT 10/27/2009: Please see the updated statement on this subject here.

Please be advised that Single Label Domain (SLD) support in the next version of Exchange is under review.

In February 2008 we announced that Exchange 2007 would be supported on an SLD. At the same time we announced that the next version of Exchange would not be supported on an SLD. 

This policy is now under review and it may be that the next version of Exchange will be supported on an SLD. The Exchange product team's view continues to be that SLDs are not a recommended configuration, and that customers should fully evaluate how remaining on an SLD impacts  their environment.  Anyone who is currently planning on migrating their SLD to a Fully Qualified Domain Name (FQDN) only because of the February 2008 announcement should also take into consideration that the policy is being re-evaluated. We expect to make the decision public in the next few months.

- Ed Beck

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Please continue this review. Many small sites have migrate up from SLD 5.0 and 5.5 environements and don't want to do a full domain migration.
Also please look into backup (with tapes)  solution. Small sites with only one Exchagne server and a couple of other servers (DC file and print servers) don't want to go the DRM route, don't have second sites to replicate with.
Quick and dirty NTBACKUP was great for disaster recovery and comitting logs in in a crisis. Tapes are cheap over 10 or 15 years, and I have fairly frequently been able to restore old data from Exchange 5.5 or 2000 backups onto reconstructed systems (build a single server and restore your tape fromt he drive you kept and the old CDs)
What's your suggestion for getting back an e-mail deleted today in 8 years time? Buy a very expensive archive system? Small shops are not going to do that.
They are not going to put their data online either.
Beware of reducing basic functionality - small companies grow!
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I'm curious to know how many AD environments Microsoft estimates to be SLD.
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I have installed Exchange 2007 in SLD environments, but when I tried to upgrade those environments to SP1, it would not allow me to continue because of the SLD. So it appears that it already does not support SLD environments.
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Lee, you need to download the latest pre-requsites check.  It will Allow SLD install.  See the above link.
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I've been involved with this issue since the origional technet forums post, and I'm curious to know what's so "bad" about SLDs from an Exchange perspective anyway.  I understand it doesn't allow for proper namespace scale, but is that all?  Seems like customers should be able to make their own decisions about that anyway.  I think the harsh warning is good, but no reason to stop people without cause.
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On the number of SLD, I support about 20 sites and 2 are SLDs. They have generally had Exchange for many years, have one or two servers and don't need or want to go through a major domain restructuring.
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We have a forest root SLD with no significant issues arising from this other than the uncertainty over Exchange, so we fall into the category described of those planning to migrate only because of the Feb 2008 announcement. Given the number of MS and other products we have which don't support a domain rename, the options are - build a whole new AD and migrate to it.  

However, having to wait for "the next few months" (on top of those we've already waited) for the possibility, however remote, that SLD's will be supported after all is playing havoc with our strategic planning.

Those of us left in this limbo have a right to be slightly embittered by Microsoft's apparent neglect of the issue.  Our AD design with SLD came from a highly reputable MS consultancy partner in 2001. It's hardly a new type of configuration and, given the serious potential impact on those affected, MS really should have been on top of it long before now.
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Flexibility and choice would lend my vote to for SLD capability, but I would recommend a clearly defined and communicated support limit document.

I would venture the goal be to make the product work in any and all ways it can? and clearly communicate ways it cannot?

SLD is a choice just like circular logging is a choice... mostly a bad choice, but in a few circumstances it is best solution for a customer.

How about a matrix of features for Exchange with a corresponding recommendation level? top 10, top 100, bottom 10

The choice on how to deploy needs to remain with the customer.
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So please provide a tool for change domain name on Exchange 2007 environment..
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I agree with Meelis. I inherited and SLD problem and would love a tool to fix it.
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I agree with everyone that it should be supported. It very easy to say to migrate to new domain but it is not easy task. It may take few years to migrate to new domain. In most cases it very hard to rename the domain because of the AD integration with several application.
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