Beware your microSD will be assimilated.

I now have three microSD cards that have been assimilated by my windows phone 7.  Once you place the card and do a reset to your device the microSD card will be unusable by other devices.  You will not be able to format it even.  I have tried to gain access to the drive in windows, mac OS X and also Linux.  The device no longer loads.

Microsoft has documentation stating "You should not remove the SD card in your phone or add a new one because your Windows Phone 7 device might not work properly. Existing data on the phone will be lost, and the SD card in your phone can't be used in other Windows Phones, PCs, or other devices.".

I personally think this is a poor implementation of standard hardware.  As a normal user I ignored any and all warnings.  For instance the Samsung Focus clearly states read the Quick Start book before installing any microSD card.  That being said it does not state that the card will be unusable by other devices after the fact.  Instead it only states: "Warning: Do not remove the memory card once it is inserted because it is integrated with the phone's memory and will cause the phone to be inoperable."

To take over a consumer purchased piece of hardware and make it unusable does not seem acceptable to me.  More to come on this front.  Hopefully a utility will be released to fix my three worthless microSD cards.

References

Microsoft Support Page
Focus Quickstart