Does anyone know of a way that I could set up an “active” history folder?
I would like to have a folder that shows the last X days files added to DEVONThink, so that I can browse them as I do the contents of any other folder. I know that I can refer to the History tool, but it only allows me to open each file externally.
Forgive me if there is a screamingly obvious solution for this that I have passed over.
The History window plus the Reveal command (Command-R) will accomplish what you want, I believe.
Select any item in the History window, press Command-R and that item will be displayed in its group view.
I use Exposé to return to the History window. The default behavior of Exposé to display all open windows in the frontmost application is to move the cursor to the upper right corner of the screen. Then click on the desired window.
Hope this helps. It should have the practical effect of a “History Group” whithout having to create a new group. 
BUT: If you really need to create a History group, that will be easy to do in DEVONthink Pro 1.9 or in DEVONthink PE 1.9. (They should be available fairly soon.) Here’s how:
[1] Create a new group and name it, e.g., History Folder.
[2] From the Tools menu, open the History window.
[3] Select a range of entries, e.g., those for the past 18 hours.
[4] Place the cursor on any selected item. Press the Control key. From the contextual menu options, select “Replicate to.” Choose the History Folder that you just created. (Sorry, “Replicate to” isn’t available in DT PE 1.8, but will be available in a forthcoming update.)
[5] You’ve got your history folder for the selected time interval. Do whatever you want with it. Then, it’s probably best to delete the folder, which contains replicants of information already in your database.