I don't know of any PTOs that reimburse gas mileage, but the IRS allows you to deduct it (and other volunteering expenses) from your taxes. Here's a link to an IRS publication on charitable contributions (it's a pdf) that details what you are and aren't allowed to deduct.
- Craig
Links in this post:
charitable contributions: http://www.irs.gov/pub/irs-pdf/p526.pdf