To integrate Notes and .NET, IBM, Microsoft and the 3rd party all provide solutions.
IBM’s Redbook web site (http://www.redbooks.ibm.com/redpapers/pdfs/redp3868.pdf#search=’IBM%20Notes%2FDomino%20.NET’) tells us to integrate Notes through ActiveX/COM, Web Service and .NET approach with the outside world.

In most cases, we want Notes database to be accessed with Active/COM, Web Service, or .NET Assembly. In other words, we want consume Notes information from the outside world, or BizNET world.

Microsoft also has a web site, http://www.msdomino.net/, to provide links of the solutions. It looks unofficial, but it has tons of demo projects and sample codes. The approaches are clearly VS.NET oriented, instead of Notes Oriented in IBM’s web Site. It can be easier, and more familiar for us to implement.

Proposition, Inc., also provide the so called N2N solution: http://www.proposion.com/site/proposion.nsf/pages/N2N, to integrate Notes with NET. It provide middle ware to make the integration easier, but it is fee based. The Server Run-Time license starts from $1595.