No, not one-line .
Just a guess on my side here a.k.a. warning!, untested stuff ahead:
<IfModule mod_rewrite*>
# * = Your rewrite module may be named differently.
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} \.aspx$
RewriteRule ^(.*)\.aspx$ $1
</IfModule>
But according to : http://wiki.apache.org/httpd/WhenNotToUseRewrite .
it might be better expressed as :
RedirectMatch ^(.+)\.aspx$ $1
Hey, wait, that's one line... Now just hope that it works... :P
If not, this might :
AliasMatch ^(.+)\.aspx$ $1
Sorry, not much of practical help at the moment, good luck.