Most SD cards won't sleep unless they have been initialized.
I am the author of SdFat which is also the core software for the standard Arduino SD.h linrary.
I've seen the quote before about the sandisk automatically sleeping when the clk signal isn't received for 5ms or more but i'm not sure if the arduino ever stops the clk signal? and if this is relevant in SPI mode?
The only time SPI clock is active with these libraries is during an actual data transfer. In your case it will be rare for SPI clock to be active.
I did another test with a 5 V Uno and and this Adafruit module MicroSD card breakout board+ : ID 254 : $7.50 : Adafruit Industries, Unique & fun DIY electronics and kits
I measured the current from 5 V on the Uno to the Adafruit module after the SD was initialized by the SdFat QuickStart example. I used a Transcend 4 GB class 6 MicroSDHC card.
The current was only 0.21 ma even though this module has level shifters and a voltage regulator. The module activity LED on SCK is not lit when the card is not active.
You don't want to power the card off and on. You then need to reinitialize the card and SD.h wrapper for SdFat has a bug that prevents multiple calls to begin().
Could you download SdFat Google Code Archive - Long-term storage for Google Code Project Hosting. and run the QuickStart example. Measure the SD current after QuickStart prints its success message.
You must have the meter connected during the entire test since the card may draw high current until it is initialized and disconnecting the card will cause it require initialization.