Items added to Inbox don’t immediately appear in the Today Smart Group when it’s been previously selected in the sidebar. They only appear after the Smart Group has been unselected/reselected. This probably applies to other Smart Groups but I haven’t actually tested it. Hopefully dynamic Smart Group refreshing will be supported in a future release.
PS: Apologies if this has been previously reported.
I’ve never noticed this before, so I tried to reproduce it and I don’t see this behavior myself.
Currently smart groups of databases are automatically refreshed, contrary to the ones in the sidebar. Just a desperate attempt to avoid another preference as we’ve received requests for both approaches 
E.g. the sidebar can be used to read unread items without having them disappear immediately after reading.
That’s an important distinction concerning what I said earlier. I was speaking of smart groups in the database. I don’t use the ‘Today’ group in the sidebar and I overlooked that sjk was speaking specifically about the group in the sidebar.
I wasn’t aware of that difference; thanks for the explanation. I’ve appropriately prepended “sidebar” in the topic’s Subject.
To me it’s counter-intuitive and inconsistent for sidebar Smart Groups not to auto-refresh while other ones do. Objects using the same name that behave differently, especially within the same app, is unnecessarily confusing. And auto-refreshing SG’s are common in other apps.
I can see the value in that, but disagree with changing SG behavior to support it.
I anticipate items under Today (and at least other date-matching SG’s) will immediately appear/disappear. One of my intended uses for the Today SG is to more easily monitor for incoming items into a relatively crowded Inbox; having to deselect/reselect it cripples that purpose.
The next beta will refresh the smart groups of the sidebar.
I believe that is a positive change. While I don’t currently use the Today smart group in the sidebar, if I did I would expect it to behave the same as does the Today smart group in a database.
Thank you!
Going back to something people liked about the non-refreshing behavior:
the sidebar can be used to read unread items without having them disappear immediately after reading.
I’ll often use Quick Look to preview items under the global Today SG, which doesn’t mark them as read and won’t cause them to disappear with the auto-refresh change. If opened in a way that’ll mark it as read, Reveal (Command-R), when possible, can be used to locate it after it’s gone from Today.
That’s a useful trick, thanks!
I do think, however, that adding an option when creating a smart group to autorefresh or not would be nice. It would enable some SGs to be set up as running searches and others as a quick way to set up a working group. Of course this can be done even with only autorefreshing SGs (set up a SG and replicate the contents to a group, maybe via a script such as the one cturner recently posted), so I guess this isn’t strictly necessary.
If that was implemented I can imagine wanting a way to easily distinguish the two types. Otherwise there’s this issue I mentioned earlier:
Objects using the same name that behave differently, especially within the same app, is unnecessarily confusing.