When it's closed, it'll also be closed to pedestrians & bikes. ODOT is supposed to start running a shuttle bus to get pedestrians & bikes across the river, via the I-205 bridge.
It sounds like the exact hours & details of that service are still TBD, but in one report,they did note that it would focus on weekday commute hours, since commuting was considered higher priority than "recreation".
I doubt you'll experience any closures this Saturday, since it sounds like the full 2-year closure won't start until January 2011.
More info at the links below.
Johnathon's report on the closure & shuttle
working group's report on the proposal