In Magento 1.5, accessing the catalogProductInfo API call from C# like this works with non-numeric SKUs:
catalogProductRequestAttributes fetchattrib = new ca
There is a workaround for pure numeric or mixed SKU, that works quiet well for me.
Just add a whitespace at the end of your SKU. Magento will interpret the value as a SKU, because whitespace is non numeric. Internaly Magento trims the whitespace later
This works perfectly from Magento 1.4.x - 1.9. (Edit: Thanks at Brett for testing with 1.9)
Example:
catalogProductReturnEntity ret = m_magentoClient.catalogProductInfo(sessionId, sku+" ", storeView, fetchattrib, productIdentifierType);