I am certainly no expert, but I highly doubt that the mini will be updated before the imac. Even then, I would not be surprised if apple waits a month or two before updating the mini. I would guess that they make more profit off of the imac, so they really would like for people to buy that instead.
As others have noted, the difference in component prices between the $599 mini and the $799 mini seems to be much less than the $200 upcharge would warrant. So I would guess that their strategy is to get people (especially switchers) looking at the $599 model - then most of them (I guess) decide that they want a DVD burner, and see that it also comes with a bigger hard drive and faster processor. So they cough up the extra $200. But they might not have even looked at apple if the cheapest computer had been the $799 model, especially since they could get a cheap Dell for less than half that.
So I think that apple likes to have a relatively low cost entry computer to get people's attention, but it is also to their advantage to keep it sigificanly inferior to their other products. That way, people will want to move up. So I would not expect mini upgrades until the imac is still clearly far superior to the upgraded model.
In my opinion, the average consumer cares about processor speed, RAM, and DVD burner first. Maybe HD capacity could be there, but with the ability to add external drives, I'm not sure if that is really a top level concern. Next is graphics card, HD speed and capacity (see note above), and ability to upgrade. Last are things like bus speed and chipset, etc.
So - I doubt that apple will make a mini that is relatively close to the imac in the top two areas of consumer interest. Right now, the processor speed is not much different. But the max ram is smaller, the HD is smaller, and the graphics card is significantly worse. If the imac receives a big bump in one of these areas, then that will create space to improve the mini while still maintaining space between the products.
That's why I don't think that we can realistically hope for a non-integrated GPU (but a jump to the x3100 is realistic).
I also think that we may be stuck with a standard combo drive in the lower end mini instead of a DVD R/W. That difference is (in my mind) the simplest way to get a buyer to pay $200 extra dollars for the higher grade mini.
I also won't be shocked if we don't see an upgraded mini until June or so, though I would love to be wrong.