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