I am a bit stumped with this one. I am doing a build using 3.0.1 and the latest Blog and Widgets modules. I had previously done this in 2.4.7 (iirc) and all was fine.
The set up is to have a widget sidebar with an archive and tag cloud widget. When I first got this all set up there were no dates on the archive widget, which I couldn't understand. I did a bit of debugging and it turns out that the Dates function wasn't even being called. I even tried putting in a dummy function and calling that... no joy.
Then I changed the order of the widgets in the Widgets area. On a reload, hey presto, archive dates. Wooo hooo! No tags. Booo!
Swapped them round again, no archive dates, but tags back.
To exclude the 'I have done something dumb' issue, I did a completely vanilla build and the same thing happens.
The screenshot shows the two widget positions, I have chucked a couple of ':' into the widget template before and after the $Content.