Looks like even Dashboard is getting in on this 'spaces' concept with MultiDash - a workspace manager for your Mac OS X 10.4 widgets. Flipping the widget over allows you to create separate workspaces, and it simply records which widgets are open, complete with a screenshot, when you create the workspace. Flipping the widget back over allows you to click the title bar to chose which workspace you want to see and potentially switch to. Clicking the button on the bottom... well, that's pretty self explanatory.
After tinkering around with this widget, I have to say: this is fantastic. Setting aside the debate about widgets and system resources, this is a killer way to manage sets of widgets for different Dashboard uses, rather than compromising with keeping all your widgets open or dealing with the less-than-ideal widget strip in the Dashboard to re-open widgets as you need them.
MultiDash is available from JBearTechnologies as donationware, and I highly recommend dropping them at least a buck (pound, etc.) or two for their slick work.