Technology platforms

If you were asked to build an application like http://basecamphq.com/, what technology platform would you use? Eg PHP, Ruby on Rails, .Net, MySQL, SQL Server, Apache, IIS etc. We know this depends on a lot of things but please just give us your first impressions.
What payment system choices would you initially recommend?
Please explain your choices above. We would like to get an idea of your preferred technologies and your expertise with them.

If we were implementing a system such as the one described we would use Ruby on Rails on Engine Yard managed Amazon EC2 infrastructure (Linux, MySQL).  Rails is a natural, proven, scalable choice for this sort of application that would enable rapid test driven development with strong security, strong standards, and manageable code. We have many thousands of hours of experience with Rails.

We are an Engine Yard Select Partner, have implemented on their platform, and know and trust their staff, and most importantly their technology stack is appropriate for a startup that wants to manage cost yet still have unlimited ability to scale. A lot of the standard technology stack elements, as well as backup and some key deployment elements are taken care of by Engine Yard so we can focus on the application. Cloud infrastructure makes a lot of sense since each individual client account is likely to be small, interaction between accounts will be minimal or non-existent, and the financial cost of infrastructure will be more-or-less linear and in line with revenue. Amazon EC2 allows on-demand deployment throughout the US, and in Ireland to serve the EU.

At a slightly lower cost point, while still maintaining strong scalability and reliability, Joyent provides scalable hosting on an Open Solaris platform on which we have a lot of experience. However, they do not have a UK data centre.