If memory serves they specifically said in the panel that mods like the minimap would be possible in the game. The specific question was something like
"Will you ever add functionality like Rei's Minimap directly into the game"
"No, but you could get a mod for that".
Oddly there seems to be confusion even in the initial post that summarizes the panel (I might be the one that is confused, though, in fact that's probably more likely, but I haven't been able to interpret the panel as saying client side mods would be impossible)... If you can stand people asking really stupid questions*, it's a good idea to give it a go if you are interested in details. I don't remember anything specific that said there would be no way to have Client-side modifications. Obviously they would be necessary since mods will be adding blocks and textures and whatnot, so they are going to have client-side assets.
On a side note, the panel was actually HILARIOUS... when people ask a dumb question you can see Dinnerbone looking off into space with a neutral smile, going to his happy place and mentally stabbing the people who constantly asked questions like "are you adding new mobs" or "Are you adding smoothstone slabs"
*Seriously... who goes to the convention to a game, get's a chance to ask the developers a question, and then asks "How long has this game been around"? An idiot, that's who.