Congratulations to the Octopart team for the fantastic work! For those wanting to try out javascriptMVC3.0 here's where you can get it: http://github.com/pinhook/framework
I think the magnifying glass is perfectly acceptable here. I think the "filter" icon is much less recognizable to the majority of users. (case in point, the author of the app hasn't heard of it).
Some things would be improved with more attention to accessibility. Ext.tree.TreePanel, for instance, allows keyboard navigation out of the box. In a similar vein, the results grid doesn't allow row selection/keyboard navigation (ref. Ext.grid.RowSelectionModel).
JMVC doesn't provide packaged widgets (though it is starting to). It's about providing a middle layer (above jQuery, less than customizable widgets) to build customized apps that are as light as possible.