I just checked this for another question: JavaScript for…in vs for.
On kangax’s ECMAScript 5 compatibility table, Array.forEach gets a ‘yes’ for all browsers except IE8.
As of September 2011, IE browser share on desktop devices is less than 40%, and at least 8% of browsers are IE 9.
In other words, Array.forEach is now supported by around 70% of desktop browsers. Obviously, this figure varies considerably, depending on territory and other factors — some regions or countries (such as Brasil) have a higher proportion of Chrome users, for example, and some (such as China) have far more users on IE6 and IE8.
I haven’t checked, but mobile support (on WebKit and Opera browsers) may be even higher.