I just found some great news for Safari 5 users. Theres a new extension called NinjaKit by a Japanese programmer 0xFF that lets you install greasemonkey scripts. Its a much simpler process to install than Greasekit, whereas greasekit required you to also install SIMBL, this extension is self contained and PC + Mac compatible, just install the Ninjakit extension and you can then click on any greasemonkey script link to install the greasemonkey scripts.
To install NinjaKit you just need to click the “Safari for NinjaKit” link (UPDATE Please note the extension link wont work in the translated page, you need to click it in the original page)
Once you’ve installed that you can then install any greasemonkey script you like, of course not all will be compatible but a lot should be.