Para aqueles que como eu habilitou a biblioteca gzip no servidor apache, certamente notará que o preload não aparecerá. Então pesquisando na Web, percebi que realmente este é um problema que existe e só é contornado adicionando uma exceção ao módulo deflate.

Veja a configuração retirado da página do mod_deflate.

<Location />
    # Insert filter
    SetOutputFilter DEFLATE

    # Netscape 4.x has some problems...
    BrowserMatch ^Mozilla/4 gzip-only-text/html

    # Netscape 4.06-4.08 have some more problems
    BrowserMatch ^Mozilla/4.0[678] no-gzip

    # MSIE masquerades as Netscape, but it is fine
    # BrowserMatch MSIE !no-gzip !gzip-only-text/html

    # NOTE: Due to a bug in mod_setenvif up to Apache 2.0.48
    # the above regex won't work. You can use the following
    # workaround to get the desired effect:
    BrowserMatch MSI[E] !no-gzip !gzip-only-text/html

    # Don't compress images
    SetEnvIfNoCase Request_URI 
    .(?:gif|jpe?g|png)$ no-gzip dont-vary

    # Make sure proxies don't deliver the wrong content
    Header append Vary User-Agent env=!dont-vary
 </Location>

Agora vamos adicionar uma exceção ao gzip, fazendo com que os SWF não mais sejam compactados.

<Location />
    # Insert filter
    SetOutputFilter DEFLATE

    # Netscape 4.x has some problems...
    BrowserMatch ^Mozilla/4 gzip-only-text/html

    # Netscape 4.06-4.08 have some more problems
    BrowserMatch ^Mozilla/4.0[678] no-gzip

    # MSIE masquerades as Netscape, but it is fine
    # BrowserMatch MSIE !no-gzip !gzip-only-text/html

    # NOTE: Due to a bug in mod_setenvif up to Apache 2.0.48
    # the above regex won't work. You can use the following
    # workaround to get the desired effect:
    BrowserMatch MSI[E] !no-gzip !gzip-only-text/html

    # Don't compress images
    SetEnvIfNoCase Request_URI 
    .(?:gif|jpe?g|png|swf)$ no-gzip dont-vary

    # Make sure proxies don't deliver the wrong content
    Header append Vary User-Agent env=!dont-vary
 </Location>

Agora o preload voltou a aparecer na sua aplicação Flex.