You cannot default the Windows Mobile E-Mail to the SD storage card (micro-SD for Treo 800W). You can have attachments for e-mails stored on the SD storage card to free up system memory.
You can change where the attachments are stored by:
Open Windows Mobile E-Mail
Select “Menu | Options”
Select the “Storage” tab
Check “When available, use this storage card to store attachments”
You can optimize how much e-mail you see from your Windows Mobile E-Mail exchange account properties. To minimize on system storage requirements, I recommend using plain text instead of HTML on the device. I also limit the automatic download to 1 KB and no attachments. You can change these settings based on your needs and the memory impact to the system. If you have limited connectivity you may want to download entire messages. If you need more than the default of 1 weeks’ worth of your e-mail on the device you can change that as well.
Many of the limitations are designed around the Windows Mobile OS memory limitations. When Windows Mobile 7 is released, a number of the current memory limitations will be resolved.
As to if Microsoft will change the Windows Mobile E-Mail to be pointed to an add-in storage card I doubt it. Doing so places the system in a state where if the card is removed, E-Mail will break. Reducing support issues is a goal of all everyone.