There are 2 types of cash considered as sideline,
1. Cash reserved by trust funds, mutual funds, etc.
2. The complete amount of money mkt funds.
For the 1st one, they are at the all time low level in terms of percentage, possibly due to the fund mgrs averaging all the way down from year 2000 til now and the prices are still below their average cost, say, on stocks like CSCO
For the 2nd one, which is quoted as cash that will switch to other fund types like bond, index, foreign, etc. is the one that has been accumulating for a while. This type of money is earning short term interest only.
So, indeed, when such money are switched to say stock mkt related funds - like those index funds, they do have the power to drive the mkt higher.
The key question is whether these money were suppose to be the cash saving that used to go into saving accounts and/or checking accounts, but due to the cost of maintaining accounts with banks is so high, has driven the public to put their regular cash reserve in money mkt fund instead. After all, the money mkt funds now have all the benefit of writing cheques, ATM banking, etc.