RM2 still allows movement when concentrating on a spell: it is just limited to 10% movement. You can also move 25% in the round when you cast the spell.
To answer JDale's question, yes, RM2 has the original rule for opportunity actions (Arms Law, red band edition, p. 8, section 3.2, 'Opportunity Action'). Declaring an opportunity action allows you to delay that action to a later phase or round. While holding an opportunity action, the only other action you can take is movement. The amount of movement you can take is limited to what you would normally be able to take had you already taken the action held -- RM2 gives the specific example of holding a spell, which limits the character to 25% movement.
Note that holding a spell allows more movement (viz. 25%) than preparing (10%), so if you have fully prepared a spell and are just holding the casting of it, you would under the RM2 rules as written still be able to move 25% until you let it go or cancelled it.