The application is planned to be built using ASP.NET, .NET Remoting & MS SQL Server. High availability is required at presentation layer, application layer and database.
Does IIS 7.0 provide any advantages over IIS 6.0 in regard to the High availability aspect?
Among the many aspects you want to consider, make sure that you have numbers.
By numbers, I mean how many request per second do you want to deliver? How many users per day are you planning? Are they all going to come in 1 hour or through out the day? Are they simply buying stuff on a e-Commerce website or is it a social network website with lots of pictures and videos?
All those questions matters in how you will architecture your website. If you go with a simple e-Commerce website that should not crash, make sure to have 2 servers with load balancing with some health monitoring on the IIS process. For the database, 1 machine will do the trick especially if you have some RAID hard drives.
However, if you go toward a social network site... things get freaky fast. If users upload pictures, you will need lots of space and much more if they upload videos. You might want to use Cloud Service to host those pictures without too much fees. For videos, you might want to use embeded link like Youtube or Google video.
As for IIS 7.0 versus IIS 6.0, I don't think there will be any significant changes. Both are really reliable.
Make sure your design scales in a horizontal manner.
That is, have your system hiding behind a load balance layer with the servers that are actually providing the service behind the load balance layer.
When you need to increase capacity, you build a new server or servers and plug it in alongside the existing servers. Then you configure the load balance layer to also consider the new server(s) when passing out the work.