I took a look around the App Store and to my knowledge there is no App as of yet that integrates Shopify with SAP (if there was it would be here: http://apps.shopify.com/categories/accounting)
If you were interested in developing an App yourself, this should put you in the right direction: http://www.shopify.ca/partners/apps
Hope this helps and feel free to e-mail me if you have any other questions or need help with anything else!
All the best,
What version of SAP are you interested in integrating with? We have a cloud based solution for SAP ERP (the R3 platform which includes ECC 6.0 etc.).
We have an out of the box solution for Magento but would love to extend it to Shopify. Especially if you're interested in using Shopify to support the B2B eCommerce scenarios.