Here is the form I created:
<r:mailer:form>

<legend>Enter your contact information and message.</legend>
<table>
  <tr><td>
    <r:if_error on='name'><p class="error">Name is required</p></r:if_error>
    <label for="name">Name</label></td><td><r:text id="name" name='name' required='true'/>
  </td></tr>
  <tr><td>
    <r:if_error on='email'><p class="error">Email is required</p></r:if_error>
    <label for="email">Email</label></td><td><r:text id="email" name='email' required='true'/>
  </td></tr>
  <tr><td>
    <r:if_error on='message'><p class="error">Message is required</p></r:if_error>
    <label for="email">Message</label></td><td><r:textarea name='message' required='true'/>
  </td></tr>
  <tr><td colspan="2"><input type="submit" value="Send" /></td></tr>
</table>

</r:mailer:form>

Now the "mailer" part:

subject: "Sent from your mailer form"
redirect_to: /contact/thanks
from_field: email
recipients:
- email@gmail.com

And the "email" part:

from: <r:mailer:get name='name'/>, <r:mailer:get name='email'/>

message:
<r:mailer:get name='message'/>

Finally, the stack trace that is printed when I submit an empty form:

/!\ FAILSAFE /!\ Sat Aug 01 10:07:29 +0800 2009 Status: 500 Internal Server Error wrong number of arguments (1 for 2)

/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rack-cache/lib/rack/cache/metastore.rb:94:in `restore_response'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rack-cache/lib/rack/cache/metastore.rb:94:in `invalidate'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rack-cache/lib/rack/cache/metastore.rb:93:in `map'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rack-cache/lib/rack/cache/metastore.rb:93:in `invalidate'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rack-cache/lib/rack/cache/context.rb:137:in `invalidate'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rack-cache/lib/rack/cache/context.rb:69:in `call!'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rack-cache/lib/rack/cache/context.rb:50:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/head.rb:9:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/methodoverride.rb:24:in `call'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rails/actionpack/lib/action_controller/params_parser.rb:15:in `call'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rails/actionpack/lib/action_controller/rewindable_input.rb:25:in `call'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rails/actionpack/lib/action_controller/session/cookie_store.rb:93:in `call'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rails/actionpack/lib/action_controller/reloader.rb:9:in `call'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rails/actionpack/lib/action_controller/failsafe.rb:11:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/lock.rb:11:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/lock.rb:11:in `synchronize'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/lock.rb:11:in `call'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rails/actionpack/lib/action_controller/dispatcher.rb:106:in `call'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rails/railties/lib/rails/rack/static.rb:31:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/urlmap.rb:46:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/urlmap.rb:40:in `each'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/urlmap.rb:40:in `call'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rails/railties/lib/rails/rack/log_tailer.rb:17:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/content_length.rb:13:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/chunked.rb:15:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/handler/mongrel.rb:61:in `process'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:159:in `process_client'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:158:in `each'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:158:in `process_client'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `run'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `initialize'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `new'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `run'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `initialize'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `new'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `run'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/handler/mongrel.rb:34:in `run'
/usr/lib/ruby/gems/1.8/gems/radiant-0.8.0/vendor/rails/railties/lib/commands/server.rb:111
/usr/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
/usr/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
script/server:3

It's not a major issue, but you might want to know this:

Attribute "name" exists, but can not be used for this element.

You have used the attribute named above in your document, but the document type you are using does not support that attribute for this element. This error is often caused by incorrect use of the "Strict" document type with a document that uses frames (e.g. you must use the "Transitional" document type)

Solution is here: http://snippets.dzone.com/posts/show/4073
I took a look at the code, but for me (not a coder) it looked like the name attribute is used all over the code, so I decided to leave it to real coders ;)