Using RGBa for progressive enhancement is getting more and more popular, which is awesome. Even nearly a year ago it was pretty much ready to rock. A great way to handle the progressive enhancement part is just to declare a fallback color before the RGBa value, so older browsers that don't support it will get a solid color version:
background: rgb(200, 54, 54); /* The Fallback */
background: rgba(200, 54, 54, 0.5);
The above works just fine, however Dan Beam alerted me to an interesting bug/quirk with this in IE 6 & 7 (fixed in IE 8). See example.
The situation is that the RGB fallback color only works when using shorthand. If you were to declare the fallback color like this:
Using the background-color property only, it will fail and display no background color at all.
The above image is from IE 7. Using the shorthand (top) succeeds while the non-shorthand (bottom) fails.
Using RGB for a fallback is nice. It's no-brainer work because all you have to do is duplicated the RGBa value, remove the "a" and remove the 4th (opacity) parameter. If you want to keep using RGB as a fallback, just remember to set it using shorthand (if possible), or set the fallback using regular HEX codes or keywords.
In the example, the result of 50% red is a light red anyway, so using a hex code to specify that value might be a more appropriate fallback color anyway.