Classic had them, they're also on the todo list. See FAQ:
[quote]What happened to (or where is) <some feature>? C2 is very young. Features will be added in time. We probably want to add that feature, but haven't got there yet. We're trying to nail down all the basic features to make C2 useful first, then we'll come back to flesh things out later.[/quote]
Collision detection with hundreds (if not, thousands) of tiny particles can be really slow. You might want to just Z order the particles so they display under other objects and naturally get hidden, rather than having tens of thousands of expensive collision checks every second.