Michael Bacon, to my knowledge email addresses are never case-sensitive. As far as I know, the structure of email addresses were intentionally designed years ago not to be case-sensitive to avoid confusion and misdirection. The use of upper and lowercase letters in email addresses is purely cosmetic and a matter of personal/institutional preference.
With regard to using the Peek with Google Voice, I've done so quite often without any difficulty at all, even though Peek changes the addresses to lowercase. What does cause problems when using the Peek with Google Voice is choosing the wrong "From" email address when replying to a Google Voice SMS email. Here's what Google Voice "SMS to email" Help says about that:
"You can only reply to SMS from the email address it was originally sent to. If you're forwarding messages from the email address associated with your Google Account to another email address and try to reply to the SMS from the other address, your SMS won't be delivered." [http://www.google.com/support/voice/bin/answer.py?hl=en&answer=160203]
So, if you have more than one email account on your Peek, be sure to choose the correct one when sending/replying to a Google Voice SMS email.
