index.haml

%html
  %head
    %title Welcome to Grails
    %meta(name="layout" content="main")
    :css
      #nav {
        margin-top:20px;
        margin-left:30px;
        width:228px;
        float:left;
      }
      .homePagePanel * {
        margin:0px;
      }
      .homePagePanel .panelBody ul {
        list-style-type:none;
        margin-bottom:10px;
      }
      .homePagePanel .panelBody h1 {
        text-transform:uppercase;
        font-size:1.1em;
        margin-bottom:10px;
      }
      .homePagePanel .panelBody {
        background: url(images/leftnav_midstretch.png) repeat-y top;
        margin:0px;
        padding:15px;
      }
      .homePagePanel .panelBtm {
        background: url(images/leftnav_btm.png) no-repeat top;
        height:20px;
        margin:0px;
      }

      .homePagePanel .panelTop {
        background: url(images/leftnav_top.png) no-repeat top;
        height:11px;
        margin:0px;
      }
      h2 {
        margin-top:15px;
        margin-bottom:15px;
        font-size:1.2em;
      }
      #pageBody {
        margin-left:280px;
        margin-right:20px;
      }
  %body
    #nav
      .homePagePanel
        .panelTop
        .panelBody
          %h1 Application Status
          %ul
            %li
              App version: 
              %g:meta(name="app.version")
            %li
              Grails version: 
              %g:meta(name="app.grails.version")
            %li
              JVM version:
              ${System.getProperty('java.version')}
            %li
              Controllers:
              ${grailsApplication.controllerClasses.size()}
            %li
              Domains:
              ${grailsApplication.domainClasses.size()}
            %li
              Services:
              ${grailsApplication.serviceClasses.size()}
            %li
              Tag Libraries:
              ${grailsApplication.tagLibClasses.size()}
          %h1 Installed Plugins
          %ul
            %g:set(var="pluginManager" value="${applicationContext.getBean('pluginManager')}")
            %g:each(var="plugin" in="${pluginManager.allPlugins}")
              %li ${plugin.name} - ${plugin.version}
        .panelBtm
    #pageBody
      %h1 Welcome to Grails
      %p
        Congratulations, you have successfully started your first Grails application! At the moment
        this is the default page, feel free to modify it to either redirect to a controller or display whatever
        content you may choose. Below is a list of controllers that are currently deployed in this application,
        click on each to execute its default action:

      #controllerList.dialog
        %h2 Available Controllers:
        %ul
          %g:each(var="c" in="${grailsApplication.controllerClasses}")
            %li.controller
              %g:link(controller="${c.logicalPropertyName}") ${c.fullName}

main.haml

%html
  %head
    %title
      %g:layoutTitle(default="Grails")
    %link(rel="stylesheet" href="${resource(dir:'css',file:'main.css')}")
    %link(rel="shortcut icon" href="${resource(dir:'images',file:'favicon.ico')}" type="image/x-icon")
    %g:layoutHead/
    %g:javascript(library="application")/
  %body
    #spinner.spinner(style="display:none;")
      %img(src="${resource(dir:'images',file:'spinner.gif')}" alt="Spinner")
    #grailsLogo.logo
      %a(href="http://grails.org")
        %img(src="${resource(dir:'images',file:'grails_logo.png')}" alt="Grails" border="0")/
    %g:layoutBody/

Is there a textmate bundle for jhaml?

Any of the Haml bundles should work, since the syntax is the same.

http://haml-lang.com/editors.html

How would I go about doing the following?

<%@ page import="net.MyImport" %>

Lines beginning with '<' go through as is, so you can do that with:
<%@ page import="net.MyImport" %>

This all assumes JHaml is operating as a preprocessor for GSPs.