Consume .NET services without Silverlight
Adobe Flex comes through
Unfortunately my application failed with a fault message: "Security error accessing url". Running the application in debug mode got me a more explicit message: "Denied due to lack of policy file permissions". Although I was using a local web server, Flex considered it to be a different domain. I had to add a crossdomain.xml file to the web application explicitly giving permission for cross-domain use. Silverlight is equally cautious, but currently gives a less helpful 404 error if it cannot find a suitable policy file.
After that, everything worked. A few lines of code later, and my Flex app could also create and delete rows in SQL Server, via same WCF web service.
Flex client, meet .NET service
In its online help, Adobe remarks that: "SOAP is often very verbose and heavy across the wire, which can result in higher client-side memory requirements and processing time."
It is likely true, and it would be interesting to test. Another concern is that large amounts of wrapper code can make debugging difficult, in the event of subtle SOAP compatibility issues. On the other hand, the ability to create Flex clients that seamlessly interoperate with existing .NET web services is a real advantage. Further, provided you don't peer too closely at the innards, Visual Studio does a good job of simplifying web service implementation.
What about Silverlight versus Flex? Silverlight 2 is in beta, and still lacks a visual designer unless you wheel out Expression Blend, whereas FlexBuilder has this built-in.
On the Silverlight side, using a WCF web service is a little easier, as you would expect from a single-vendor stack, and I suspect its performance is better. That said, and leaving aside the large amount of wrapper code, which Flex generated, the code in my simple client ended up similar in both.
Given that I was initially skeptical whether this would work at all, it is a good result for Adobe's client; it also suggests that for many projects either one will do.®