Ben Ryan, a developer on the version control team, has written a couple of posts about the offline feature introduced in Team Foundation Server 2008 Version Control. The first one in particular answers some questions that have come up.
TFS 2008 adds improved support for "offline" scenarios. This blog post identifies the scenarios under which a solution will be placed in "offline" mode.
For some users it may be a bit offputting that taking one solution offline causes all other solutions bound to that same server to go offline when they are opened (assuming no "Go Online" operations in the interim).