um. there is no gap. the iMac sits between the mini and the macpro. for the MAJORITY of people the imac is a great machine to surf, email, make home movies, blogs yada yada yada. they come with displays up to 23".
iMac: Dual core, low power (35W), Merom compatible chipset. This means it's comparatively slow; 667MHz bus, max of 2.33 GHz, expensive parts despite lack of speed.
Mac Pro: Soon to be 8-core. Huge power requirements (250ish Watts). Woodcrest/Clovertown compatible chipset. This means very fast bus, lots of fast cores, either 4x 3Ghz or 8x 2.67Ghz. Very expensive parts, cannot use normal RAM, only super expensive ECC.
Mid range tower: Dual or quad core, mid to high power requirements (65-130ish Watts) Conroe/Kentsfield compatible chipset. This means very fast bus speeds, and a few extremely fast cores 2x 3.2Ghz or 4x 2.67Ghz. Very cheap parts. If I remember, a 2.4Ghz Conroe is roughly half the cost of a 2.33Ghz Merom. Cheaper CPU, RAM, motherboard, etc than either Merom or Woodie compatible chipsets.
So... Mid range mac would be cheap. Very cheap. Much cheaper than an iMac, which should enable Apple to put in a better CPU, and would also enable the use of a really decent graphics card. If the rumours are true, and Apple will use their custom cooled X2800XTX GPUs, there would be every likelyhood one (or two, as the rumour also states) would find its way into a mini tower. Personally, I still think we will have to wait at least until Penryn to see the mini tower, if not Nehalem.