Hi,
As i Know there are two options to integrate BizTalk with SAP. Recently i integrated heaps of stuff from BizTalk to SAP and vice versa. Here are the options
1. If you want to integrate BizTalk directly with SAP without using FILE/webs services then you can use WCF-SAP adapter.
2. if SAP people exposes any functionality using web service end point then you can WCF-Custom adapter just like consuming all other web services.
I used the above two methods and be careful with the second option especially because in our case the end point on SAP side is exposed as asynchronous and also it supports reliable messaging protocol. So if you want to do some thing with response then you
wont get that immediately. Insteade after processing in SAP they will post the response to any end point that we expose and you have to use that in orchestration with the help of correlation.
Hope that gives better idea.
Cheers