Objective-J and Cappuccinno
On the other hand, if you're already familiar with Cocoa but lacking web skills, well Objective-J and Cappuccino certainly seems insanely compelling. 280North's flagship application (280 Slides) already developed with this set of tools is extremely fluid and attractive. Also, their scrapbook tutorial and sample application make for a great read; a lot of thought has clearly been put into this and it shows.
If you're an Objective C developer I'd definitely encourage you to take a look. If you're already comfortable with jQuery, HTML, and CSS, it may be somewhat more difficult to make the argument to switch, but taking a good read through their tutorials may still be worthwhile. I certainly can't argue with the fact that the number of technologies that we have to be familiar with as web developers is probably much greater than it should be, and it would indeed be nice to embrace a single toolset rather than 3 or 4 inter-related ones. Now if the syntax just looked a little more like Ruby, and a little less like Objective-ive-C...