- IE9 and lower do not support gradients at all.
- All browsers but Opera support prefixed middle syntax.
- Opera supports prefixed middle syntax only for non-repeating linear-gradients. (Don’t ask me why.) For these gradients, it supports the -webkit- prefix in addition to -o-.
- All browsers but the WebKit-based ones support unprefixed new syntax.
- For linear-gradients, Firefox also supports prefixed new syntax.
- Occasionally, unprefixed middle syntax is interpreted as old syntax. Since you should never use unprefixed middle syntax anyway, this is mostly an academic problem.
You can read more about CSS gradients, their potential compatibility issues and cross-browser support at PPK's website and the following references.