Extensions are coming for chrome.
Google’s Matt Cutts in a blog post says: “Extensions currently have very Greasemonkey-like functionality: you identify which web pages should be modified, plus JavaScript to be added to those pages. By default, the extension’s JavaScript runs after the page loads, but you can specify that the extension’s JavaScript should run before the page loads. Right now, you can only load one JS file, but that could change in the future. You also can’t currently load Cascading Style Sheets (CSS), but that might also change.”
Personally, I think they should just port greasemonkey directly so chrome can take advantage off all the existing scripts