Looks like iOS 5 will support the iPhone 3GS. Here are all of the leaked links that may not be live yet.
iOS 5 Beta For Apple TV
http://adcdownload.apple.com/ios/ios_5_beta/apple_tv_software_beta.dmg
Click to access installing_apple_tv_software_beta.pdf
iOS 5 Beta For iPhone 3GS
http://adcdownload.apple.com/ios/ios_5_beta/ios_5_beta__iphone_3gs__9a5220p.dmg
http://adcdownload.apple.com/ios/ios_5_for_it/ios_5_beta__iphone_3gs__9a5220p.zip
iOS 5 Beta For iPhone 4
http://adcdownload.apple.com/ios/ios_5_beta/ios_5_beta__iphone_4__9a5220p.dmg
http://adcdownload.apple.com/ios/ios_5_for_it/ios_5_beta__iphone_4__9a5220p.zip
iOS 5 Beta For iPhone 4 CDMA
http://adcdownload.apple.com/ios/ios_5_beta/ios_5_beta__iphone_4_cdma__9a5220p.dmg
http://adcdownload.apple.com/ios/ios_5_for_it/ios_5_beta__iphone_4_cdma__9a5220p.zip
iOS 5 Beta For iPod Touch 3Gen
http://adcdownload.apple.com/ios/ios_5_beta/ios_5_beta__ipod_touch_3rd_generation__9a5220p.dmg
http://adcdownload.apple.com/ios/ios_5_for_it/ios_5_beta__ipod_touch_3rd_generation__9a5220p.zip
iOS 5 Beta For iPod Touch 4Gen
http://adcdownload.apple.com/ios/ios_5_beta/ios_5_beta__ipod_touch_4th_generation__9a5220p.dmg
http://adcdownload.apple.com/ios/ios_5_for_it/ios_5_beta__ipod_touch_4th_generation__9a5220p.zip
iOS 5 Beta For iPad
http://adcdownload.apple.com/ios/ios_5_beta/ios_5_beta__ipad__9a5220p.dmg
http://adcdownload.apple.com/ios/ios_5_for_it/ios_5_beta__ipad__9a5220p.zip
iOS 5 Beta For iPad 2 WIFI
http://adcdownload.apple.com/ios/ios_5_beta/ios_5_beta__ipad_2__9a5220p.dmg
http://adcdownload.apple.com/ios/ios_5_for_it/ios_5_beta__ipad_2__9a5220p.zip
iOS 5 Beta For iPad 2 GSM
http://adcdownload.apple.com/ios/ios_5_beta/ios_5_beta__ipad_2_gsm__9a5220p.dmg
http://adcdownload.apple.com/ios/ios_5_for_it/ios_5_beta__ipad_2_gsm__9a5220p.zip
iOS 5 Beta For iPad 2 CDMA
http://adcdownload.apple.com/ios/ios_5_beta/ios_5_beta__ipad_2_cdma__9a5220p.dmg
http://adcdownload.apple.com/ios/ios_5_for_it/ios_5_beta__ipad_2_cdma__9a5220p.zip
Xcode 4.2 and iOS SDK 5 Developer Preview
Click to access xcode_4.2_developer_preview_readme__snow_leopard.pdf
Click to access xcode_4.2_developer_preview_readme__lion.pdf
http://adcdownload.apple.com/Developer_Tools/xcode_4.2_developer_preview/xcode_4.2_and_ios_sdk_5_beta__lion.dmg
http://adcdownload.apple.com/Developer_Tools/xcode_4.2_developer_preview/xcode_4.2_and_ios_sdk_5_beta__snow_leopard.dmg
iTunes 10.5 Beta
http://adcdownload.apple.com/ios/itunes_10.5_beta/itunes_10.5_beta__mac.dmg
http://adcdownload.apple.com/ios/itunes_10.5_beta/itunes_10.5_beta__windows.zip
http://adcdownload.apple.com/ios/itunes_10.5_beta/itunes_10.5_beta__windows_64.zip
http://adcdownload.apple.com/Applications/itunes_10.5_beta_for_windows/itunes_10.5_beta__windows_64.zip
http://adcdownload.apple.com/Applications/itunes_10.5_beta_for_windows/itunes_10.5_beta__windows.zip
Safari 5.1
Click to access safari_5.1_developer_preview_notes.pdf
http://adcdownload.apple.com/internet_and_web/safari_5.1_developer_preview/safari_5.1_developer_preview_snow_leopard.dmg
http://adcdownload.apple.com/internet_and_web/safari_5.1_developer_preview/safari_5.1_developer_preview_windows.zip
http://adcdownload.apple.com/internet_and_web/safari_5.1_developer_preview_windows/safari_5.1_developer_preview_windows.zip
http://adcdownload.apple.com/internet_and_web/safari_5.1_developer_preview_uninstallers/safari_5.1_developer_preview_snow_leopard_uninstaller.dmg
http://adcdownload.apple.com/internet_and_web/safari_5.1_developer_preview/safari_5.0.6_developer_preview_leopard.dmg
http://adcdownload.apple.com/internet_and_web/safari_5.1_developer_preview_uninstallers/safari_5.0.6_developer_preview_leopard_uninstaller.dmg
iPhoto 9.2
http://adcdownload.apple.com/Applications/icloud_beta/iphoto_9.2_beta.dmg
iCloud
http://adcdownload.apple.com/Applications/icloud_beta/icloud_control_panel_for_windows.dmg
http://adcdownload.apple.com/Applications/icloud_beta/icloud_for_mac_os_x_lion.dmg
Click to access copying_data_from_mobileme_to_icloud.pdf
Mac OS X Lion Developer Preview 4
http://adcdownload.apple.com/Mac_OS_X/mac_os_x_lion_developer_preview_4/server_admin_tools_11a480b.dmg
http://adcdownload.apple.com/Mac_OS_X/mac_os_x_lion_developer_preview_4/storekit_framework_for_mac_os_x_lion.dmg
http://adcdownload.apple.com/Mac_OS_X/mac_os_x_lion_developer_preview_4/windows_migration_assistant_setup.zip
http://adcdownload.apple.com/Mac_OS_X/mac_os_x_lion_developer_preview_4/mac_os_x_lion_developer_preview_4_seed_note.rtf
http://adcdownload.apple.com/Mac_OS_X/kernel_debug_kit_mac_os_x_lion_developer_preview_4_build_11a480b/kernel_debug_kit_mac_os_x_lion_developer_preview_4_11a480b.dmg
Is everyone dumb? Every website has the same links and everyone is wondering when they will be live. The thing is they are! These are developer links, which means you have to have a developer account to login with to download the software! These are hardly leaks because you have to be logged in to get them, no matter what no one will be able to download them via these links without being logged into a paid developer account. Hence the session expired error.
these were leaked before the keynote and developers could log in.