Rd vs Mt
If I am setting someone up with a road bike for the first time and they already have mt bike pedals I usually encourage them to stick with their current setup for a while. The immediate disadvantages outweigh the advantages for a newbie road rider. Having an unfamilar pedal system can just complicate the learnig process of owning your first road bike.
First and foremost walking will always be easier with a recessed cleat that is common on any my shoe. This is especially important when stopping as road cleats give very little traction. Most new riders come to a sliding stop on road cleats.
On a longer ride you may find that the smaller point of contact that a Mt bike cleat offers may cause some numbing and discomfort. Additional discomfort may also come from the lateral rocking that is more common in mt pedal than in Look and SPD-SL (Road) pedals. This is also common in speedplay road pedals, especially as the cleats wear.
If you must use SPD type pedals you might try moving the cleat back toward your arch. In most cases this reduces numbness and in one bike fitters opinion it can produce more power as well as diminished leg soreness:
Steve Hogg has a lot to say about this topic. read away:
http://www.cyclefitcentre.com/pdf final docs/SHOE AND PEDAL ARTICLE 8_final.pdf
I am not sure if I believe all that he says but I am trying his recommended position and I think he may be right.
The bottom line is that on a road bike the things that hardly bother you on the mt bike will soon drive you nuts. You have little to think about while riding other than how the click in your BB is driving you crazy or how it always seem like you are reaching for the hoods.
sorry to ramble..
j-