Which brings us around to the current hotness, Rich Internet Applications (RIA). Ever wonder why Adobe and Microsoft are slugging it out for that space? And Sun is running along behind with JavaFX saying "Wait, we want to fight too!". It's the new platform play. We've dealt with the pain of writing good looking web applications so long that when someone comes along and shows pretty pixels, we swoon. Yes, you can create beautiful applications using Silverlight and Flex. And Sun showed some awesome demos of JavaFX at JavaOne. But, if you write an application in one of those tools, you've bought a platform. You are no longer in a standards space. You can't take a Silverlight application and port it to Flex without a rewrite. Same goes with JavaFX. Whoever wins the RIA war has the new dominant web platform, just like Win32 back in the day. Sounds like a good reason for big companies to pour resources into the effort.
Of course, as my friend Mike Nygard pointed out during several expert panels, you pretty much buy into a platform play every time you start writing software. Yes, JEE is a standard across many application servers, but how many people are really careful to isolate the seductive goodies that come with your application server from the real JEE stuff? My guess is not many. So, you've kind of implicitly bought into a platform anyway. That's doubly true for .NET: yes, Mono exists, but more of a proof of concept than anything else.
Maybe this is OK. But I remember the reason to avoid single vendor platforms. One of the compelling reasons to move to Java when the JEE specs came out was the possibility of moving to another vendor, even with some strenuous effort. I can't tell you have many consulting clients I encountered in the mid-90's who had bought into a particular platform (say, Microsoft Transaction Server or ColdFusion) and were very productive with it...until they hit a real bug in the platform itself. I consulted at one company who had an 18 month project written to a proprietary platform when they encountered a show-stopping bug. They contacted the vendor who said "Yeah, that's a known bug. We're going to get a fix out for that in the next release, which should be in about 12 months". Even though it took them a long time, they scrapped what they written and started over, in Java. And you can bet they were careful about isolating the application server stuff from the specification.
Building to a single platform isn't necessarily a bad thing, as long as you go in with your eyes open. It's a trade-off: perhaps the pretty dancing pixels in Silverlight are worth the chance that you'll find something it doesn't support and end up with an application that doesn't quite do what your users want. But, beware of Dietlzer's Law: users want what they want, and are grumpy if you tell them "no". And, if you go with one of the RIA platforms, get ready to marginalize everyone without a PC. Some of these platforms have some support on mobile devices (and surely more to com, in a massive wave), but it's not going to be as good as plain old HTML for a long time.
A slightly different version of this rant, more Silverlight focused, appears here. I was at a party at a friends house that had lots of .NET folks there, during the Microsoft MVP Summit. I was getting some impending dead-line work done and Eric was recording a series of podcasts, the tone of which can be summed up by "Silverlight will cure cancer, solve world hunger, and paint your kitchen for you". After he was done, I was quietly fuming and asked him if he wanted a contrarian view. That's what's on the podcast. You've been warned!