<p>Can anyone point me to a good introduction to coding against the paypal API?</p>

I would suggest you start by downloading the SDK:
https://www.paypal.com/IntegrationCenter/ic_sdk-resource.html
The SDK includes the following:
Client libraries that call PayPal APIs
API documentation for SDK components
Sample code for Website Payments Pro and various administrative APIs
Testing console that can verify connectivity to PayPal and submit API calls
You may also want to take a look at
Encore Systems .NET* Class Library for PayPal SOAP API