There is some new code in v1.5, specifically the method get_addon_info in gravityforms.php (line 862) which gets called whenever another plugin is being installed via the WP admin, by searching and clicking the 'install now' link.
My problem is that when I install any of the extra 3rd party gravity forms plugins (including Gravity Forms + Custom Post Types) the get_addon_info method intercepts the install because the plugin slug contains the text "gravity". It then makes a call to your GF servers to verify the addon. The problem is these free 3rd party plugins are obviously not registered on your server and therefore you get an error "Install package not available" which aborts the install.
I had a look at all the official addons, and if the code is changed to rather look for "gravityforms" instead of just "gravity" it will allow for the seamless install of 4 out of 5 of the free 2rd party plugins out there.
A manual way around this is to upload the 3rd party plugin folder to the server and activate it.
Another work around is to activate the addons first and then gravity forms last.
But these just seem like hacks. Can we not come to a reasonable solution that suits us both?