Depends on how you expose the SD card to the computer. If you let it touch the filesystem directly, then yeah, the device can't use the card at the same time. But if you make the phone act like a USB network card, and then run Samba on that network interface, both devices can modify the data at the same time. (My Archos/Android mp3 player does this.)

There is also some media sync protocol that Microsoft invented that avoids this problem.