I have little experience with silverlight development still but with the potential of silverlight (office 2010 live) and similar adobe air apps are there any reasons for not building your business apps for web using silverlight.
As I see it building business apps using html and javascript is only an ugly workaround for enabling apps running on web
A download is required to get things going. You may alienate a portion of your market simply because you require a plug-in. Of course, the same thing can be said of requiring JavaScript....
Certainly if you are not availing yourself of some of the Javascript and HTML goodness that is jQuery and jQuery plugins, web development can seem very clumsy. But there are a lot of cool widgets now available in this area, and web pages can now look very rich indeed.
Given that you can now create web apps fairly gracefully there will be some that say Silverlight is overkill for business applications unless you need something fairly media intensive like a photo carousel or deep zoom.
Here is an example of what you can do in a web page with the right tools:
IMO, the answer is "it depends".
Microsoft has the clout to drive adoption of Silverlight for most small and medium businesses. This will happen sooner rather than later IMO.
Large enterprises are different. They have IT departments which lock down PCs so users cannot install even relatively simple updates like Silverlight until IT says it's OK. Some of these companies will take years to adopt Silverlight.
Some applications, such as gmail, work pretty well using JavaScript / HTML. On the other hand, I would hate to have to use the Google Docs spreadsheet for anything more than sharing small lists on the web. Silverlight overcomes the various limitations which lead to the Google Docs spreadsheet not being better than it is.
Silverlight is a non-standard extension to the web which has historically been about cross-platform effort. AFAIK you will have to use the MS runtime and can't write your own as you can with HTML, CSS and JS (I know there is Moonlight, but that's just reverse-engineered really). Them being cross-platform is the reason that every mobile device and netbook can do HTML, CSS and JS but the coverage for Sliverlight and Flash is very sketchy in this sector.
The main disadvantage of using Silverlight is that it requires the user to install a plugin (so does Flash/Air). Most users won't have a problem with this, but businesses with managed IT systems may require the IT department to test and approve the use of the plugin.
It also depends on your particular scenario. If you're a consultant creating bespoke software for a company, then installing Silverlight will be part of the requirements for deployment. Also, from your question it sounds like you're an experienced .Net developer with relatively little Javascript/Ajax experience. If time==money and you're already proficient on the .Net platform then you have to include this fact into your decision.