Apparently, the actual laws regarding when a kid can go from a car safety seat to a booster seat are pretty vague. It just says if the child is under 8, the child has to be in an "appopriatate" (read approved and manufacturer's instructions are being followed) car seat of some sort:
http://www.peoples-law.org/node/695
Recent changes to the law did not actually include the amount of time that a child has to be rear-facing, they just increased the time a car-seat must be used to 8 years:
http://articles.baltimoresun.com/2012-04-10/health/bal-child-safety-law_1_safety-seat-md-lawmakers-doctors-group
From the state itself:
http://fha.dhmh.maryland.gov/ohpetup/kiss/sitepages/mdlaw.aspx
In the FAQ, somebody points out what complete garbage "Booster" seats are (most do not attach to the car in any way):
http://fha.dhmh.maryland.gov/ohpetup/kiss/SitePages/faq.aspx
Also notable from the FAQ is apparently I've broken the law by installing the rear-facing seat using both LATCH and the seatbelt.
So, a precious few booster seats do have LATCH straps, like this one:
http://www.amazon.com/Britax-Parkway-Booster-Seat-Nutmeg/dp/B0047O3Y2G/ref=sr_1_1?ie=UTF8&qid=1352039944&sr=8-1&keywords=booster+car+seat+with+latch+system
But not this one:
http://www.amazon.com/Britax-Parkway-Booster-Seat-Onyx/dp/B002BH3NEU/ref=pd_sim_ba_2
Or this one:
http://www.amazon.com/Graco-Highback-TurboBooster-Seat-Spitfire/dp/B004GEC8LU/ref=pd_sim_ba_3
Here is a nice chart showing the transition points between each kind of car seat:
http://www.toysrus.com/info/index.jsp?categoryId=2279366&infoPath=2279362#E
