Due to electronegativity differences, the iodine will be electrophilic and the chloride nucleophilic. Correct me if I am wrong here. My question is one of regioselectivity: will it proceed through an iodinium ion (like a bromonium ion which is observed when Br2 is added) where the iodine and Me groups are on opposite faces. Then, chloride attack at the most substituted carbon? Or would it be a more akin to a planar tertiary carbocation, so attack at either face almost equally probable? **chlorine should be "back" in first structure
Iodine monochloride is usually highly specific for anti addition to alkenes. While bromonium ions form more readily than do iodonium ions, the latter do form. Additionally, the large size of the iodine atom further reduces the chance of syn addition. To answer your question in particular, it is unlikely that a planar carbocation is formed, and the anti-addtion would be expected to be the main product.
The more typical issue with iodine monochloride is that while the chloride is typically added to the Markovnikov carbon, the regioselectivity is sometimes relatively mediocre. For example, the addition of ICl to propene gives 2-chloro-1-iodopropane and 1-chloro-2-iodopropane in a 69:31 ratio; the yields of the expected regioisomer are better when styrene is used, giving approximately 95% of the expected 1-chloro-2-iodo-1-phenylethane. (J. Chem. Soc., 1931, 2742) This is likely due to the greater stability of the benzylic stabilization of the cation.