No Merom for mini...
There are other systems Apple will upgrade before the mini...
In particular, Apple will upgrade the iMac with a Merom or Conroe, and Merom will go into the MBP before it goes into the mini.
While it is feasible that Apple could do all these upgrades at WWDC I somehow doubt it.
This all assumes that Apple _will_ upgrade the Mac mini. The introduction of
the Education iMac with Intel GMA900 graphics tends to lend weight to the argument that Apple will continue to use the built-in graphics for the lower-end models, and I think Apple could leave the MacBook and Mac mini with the current Core cpus (Core Duo and Core Solo).
I would accept that it is certianly possible that Apple will drop the Core Solo from the entry-level Mac mini, making Apple first to deploy Dual-core CPU's accross the board.
Apple _could_ have one last surprise for us... I think there is definately space in the Apple line-up for a small desktop system. It would use the Conroe chip. The "Mac Pro" will use the recently released Woodcrest Xeon 5100 series CPUs.