SanDisk has a mail-in rebate offer through June 29th on several popular models of SD, CF and Memory Stick Duo cards. The rebates are on a sliding scale; the more you buy the more you save. The amounts range from $5 off a single 1GB SD card to $300 off three 16GB CF cards.
For example, I just ordered two 8GB SanDisk Extreme III SDHC cards for $74.95 each. Adorama's website lists the price as $34.95 after rebate, but that's for one. You get another $5 off each card when you buy two, lowering the per-card price to $30.95.