I had the same problem for a Dimension 4600, ordered a replacement part from Dell Parts - 800.357.3355, make sure to tell them it's the POWER ON/OFF switch or they might send you the I/O panel for the USB cables on the front panel.
It's $9.95 or so US plus shipping and easy to replace on the front panel and plugs in to the mother board, all easy to see.
Michel Ditlove
Michel Ditlove, January 2007