The Federal Reserve is working on speeding up the ACH network. Starting in August same-day ACH will start going into effect based on my understanding.
there is still a 48 hour window for transactions to be returned because of insufficient funds. and same day ACH will only be supported by a small subset of banks (and none of the big ones, at least for a long while)
the network is based upon nightly batch file exchange. it's a terrible system for 2010. but it was not bad for its time, and hard to change the system implemented by thousands of banks.
also, there is a risk component of it. holding transactions for longer decreases risk.