Why do you want your application to lie about the user agent?

It's not for lie, but for identify the application (here Mozilla is a bad exemple). 

It probably would be a good idea for Google to include the name of the application in
the user agent request header.

For exemple, like this: 
AppEngine-Google; (+http://application-name.appspot.com)

Some websites check the user agent and return different results or no results 
depending on the user agent. To support these websites, applications should be 
allowed to override the user agent. However, to prevent harvest-bots, "AppEngine-
Google; (+http://application-name.appspot.com)" should be appended to the end of the 
user agent.

Google websites "check the user agent and return different results or no results 
depending on the user agent". I am reading from the "Web Pages" optional service in
Google Apps. From the dev server it correctly returns me a normal web page. When I
uploaded the application I discovered that the retrieval returned a much simpler HTML
document with a DTD of "//WAPFORUM//DTD XHTML Mobile 1.0//EN"
"http://www.wapforum.org/DTD/xhtml-mobile10.dtd".

I am stumped. I cannot continue with this design unless I can retrieve the HTML
created from Google Web Pages.

This is also a problem for applications that want to mash up results from sites that cloak their XML based on the 
User-Agent.  Many sites will transparently apply an XSL to the XML if the User-Agent is not in a list of supported 
(i.e. modern) browsers; this makes it impossible to use App Engine to retrieve these pages.

I agree that adding an AppEngine suffix would be appropriate.  Also, this behavior is different on the Dev Server, 
which is confusing and misleading for developers; the behavior should be harmonized.

I agree. This problem makes my application almost impossible to implement because the
website I am fetching automatically applies the XSL if it doesn't detect a modern
browser. This makes what should be an easy task of parsing an XML file needlessly
difficult. I am seriously considering other hosting options because of this. 

Currently, Google App Engine doesn't allow various headers to be set:
ACCEPT_ENCODING,

Outbound request headers:
CONTENT_LENGTH, DATE, HOST, REFERER, USER_AGENT, VARY, VIA, X_FORWARDED_FOR

Response headers:
CONTENT_ENCODING, CONTENT_LENGTH, DATE, SERVER, TRANSFER_ENCODING

Allowing an application to modify the user agent is a feature request

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Is allowing an application to modify the "user agent" a feature request?  I would 
find this tremendously valuable -- What might be the timing on this?

Agree we need this.  I would ask what security concern there is in spoofing user-agent....all modern browsers 
allow this capability.

I need this feature because i want to get the apple-touch-icon link element from a website when it's 
available....most sites will only include this html element when the user-agent is MobileSafari (e.g. google.com 
and apple.com) 

We've implemented this and it's now live. :)
http://googleappengine.blogspot.com/2009/04/sdk-version-121-released.html

Setting a User-Agent of 'Foo Bar' will result in urlfetch sending the following User-
Agent:
Foo Bar AppEngine-Google; (+http://code.google.com/appengine)

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