All external SD/MMC/CF-Card/Memory Stick readers are USB devices and work fine with the usb-storage module. The only caveat which might occur is that you may have difficulties to determine the device assignment. Just use dmesg after you have connected the reader. The command should show a SCSI device like /dev/sda1 assigned to the USB drive.
Currently there are three kinds of devices available: USB, PCMCIA and PCI devices.
USB devices are seldom, but usually work out of the box. They behave like the external readers mentioned above.
Some readers are PCMCIA/CardBus devices. Often such a reader is located near the CardBus slot. The command cardctl ident will reveal these cards.
For some laptops and notebooks a driver for the Winbond's W83L518D and W83L519D SD/MMC card reader is available.
Some proprietary devices are not yet known to work with Linux. Except the readers built into the SHARP Linux PDAs, but the driver is closed source and available as a binary only for the ARM CPU.