In some sense if the customer's seriously considering a .NET solution you've already lost the battle, because the customer is not concerned about
1. lock-in
2. ever-increasingly onerous licensing terms including near-mandatory updates, etc
3. reliance on X86 hardware
4. licence audits more akin to a car-jacking than anything previously seen in the software world.
5. Mono? MS is patenting key components of .NET
6. security. Pointing out their history isn't FUD. Yes, they've claimed to be paying more attention but there've been a couple of nasty rounds of exploits since and no sign they're letting up.
7. If you want to live in the X86 space, 2K requires more resources than a headless Linux server box, and Linux wins hands-down in reliability.
There's a *very* good business case for staying as far away from MS as possible. Hear those footsteps running down the corridor? Those are IT managers running towards Linux-based solutions offered by real-world companies like IBM.
OpenACS - and some other potential solutions - really allows you to scale your solution to your problem. Are you large and feel you need the world's most popular enterprise-class RDBMS and want the security of running it on the most popular hardware platform for hosting it in the enterprise? We support Oracle and Solaris, and use the same webserver as the busiest web site in the world, aol.com. We also support vanity blog sites running Postgres on cast-off PCs.
I've used Visual Studio professionally back in my compiler-writing days, as recently as four years ago. It's a decent development environment and we don't offer anything like it. I'm sure that it's just as great for writing .NET apps in C# or VB as it is for C++. I'm sure that .NET's wide set of classes help one cobble together apps quickly.
As far as XML being as important as the PC or the spinning electron or whatever, remember that XML - like HTML - is simply a subset of SGML, which has been around for a very long time ... that's hype and nothing but hype and even if it weren't hype, MS isn't alone in using XML. You could point out that OpenOffice and KDE have used publicly-defined XML formats to store documents for years, now, since before Steve Ballmer learned how to pronounce XML.
I think there are plenty of ways you can attack an MS solution without referring once to OpenACS ... which then becomes a bit like the icing on the cake.
But if they're sold on MS then they're sold on MS and your sales budget is probably better spent on the increasing number of people who are seriously considering non-MS server solutions *without* being prodded by a vendor.
