I think there may be some misinformation going on in this thread, that or i'm just not following correctly.
First off MSHP is a non-repeated system in all the troops that i've been in. 2nd off, the dispatch centers have antennas which are hundreds of feet in the air, thus they can hear everything even though we can't. Lowband, by the physics of the band, carries further or covers more distance than say 800mhz thus you don't need that many receive sites to cover such a large area. 3rdly, The remote towers are only used when the troopers are close to them, just by monitoring 1 remote tower on UHF doesn't mean you will hear every trooper who is transmitting within that troop. The towers are setup on the edges of each troop to help cover the deadspots, and I don't believe these remote towers are receiving from lowband frequencies, if i recall correctly they are receiving UHF frequencies and then sending them via highly-directional antennas back to the dispatch center. I've never got real in-depth with all the radios that they have in the troopers cars, but it is pretty ridiculous. Atleast 1 lowband radio, 2 VHF-HI radios, a mobile repeater(usually moto pac-rt's) plus CB and who knows what else. According to the DB they also have UHF radios to communicate with the remote towers.
Further, as btritch noted, there are output frequencies used by dispatch to talk to troopers, there are input frequencies used by troopers to talk to dispatch, and then the troopers will also use the output frequency as a "talk-around" or "car to car" frequency. They will also use 42.00 as a car to car channel and for speed enforcement, as well as Law Mutual Aide, Sheriffs Net, and several other VHF-HI band frequencies to communicate. They also have access to most county radio systems assuming they are on lowband(which is only a couple at most) or VHF-HI.
I know there have been bids put in to put a new radio system in place although who knows when anything will be approved. I believe the FCC has set a 2013 deadline(was 2009) for all radios to be upgraded to digital format. I have also heard that testing has begun to figure out what kind of system will work best statewide, it sounds like it will be a multi-band system for the varying terrain across the state, but i doubt anyone really knows at this point. Regardless it would takes several years to erect towers, upgrade dispatch centers, and equip all troopers with brand new equipment. I wouldn't be expecting anything before 2010 personally, and even then they will more than likely still be using the old system along with the new. Last time I went through Illinois i was still picking up ISP traffic on lowband, and you still see some of their patrol cars with the lowband whips, and they've been in the process of getting their digital system up for quite some time.