The Time Release plugin comes close, but has a fixed update period. The functionality is there, though, and I’d guess the portion of code that does the checking/updating could be hacked to use whatever (still-fixed) frequency you want without too much trouble.
EDIT 2.16.12 Found a couple others that come much closer to Tumblr’s behavior while working on something recently.
Automatic Post Scheduler provides a simple setup where you specify a minimum and maximum interval between items. When something comes up for publishing, the system checks against the intervals and automatically pushes its scheduled time to the next available slot.
Auto Schedule Posts has a much more robust set of options, allowing you to actually replicate the sort of “every # hours between x and y times” queuing that Tumblr does. It can also be limited to only post on certain days and so some randomizing.