Question: In my silverlight application I am trying to get the actual dimensions of a png image that I am loading into an image control. 

After scowering through the internet for a while I learned that in silverlight 3 there is an attributes called PixelHeight and PixelWidth.

So I was wondering if it is feasable to upgrade to Silverlight 3 since it is in beta stages, don't want my application to be suddenly riddled with errors. 

Note I am doing this project for a school assignment so versioning of silverlight doesn't really matter.

Answer: 
Even though Silverlight 3 is expected to be backwards compatible to Silverlight 2, expect to run into some issues. The conversion wizard does a good job but you cannot completely trust it.

why don't you try it and see if you it is seemless in your case?
I've been using SL3 for a few weeks and haven't had any real issues. Some gotchas I ran into are that assembly caching isn't supported for out of browser apps and ADO.NET Data Services doesn't work out of browser (use .NET RIA Services instead). So far so good!