This weekend I was adding improved extension support to the Athena engine for iOS. There is a page on the Apple site that tells what device has what extensions but it hasn't been updated since iOS 4.2.
So I ran through each version of GLES on a 1st Generate iPod Touch (MBX), an iPhone 4 (SGX) and an iPad 2 (A5), printing the returned string from glGetString( GL_EXTENSIONS ) for each device.
There are a some new extensions in iOS5, some of which are iPad2 specific. But you can see from the image, each newer device supports all the previous extensions.
I haven't had chance to run it on an iPad1 yet, so it will be interesting to see if there is any difference. I've only wrote a rough idea of what version of what SDK is required to use them.
Anything with '***' does not need the extension to work and has an implementation for that version of OpenGL ES.
GL_OES_depth_texture GL_OES_blend_equation_separate GL_OES_blend_func_separate GL_OES_blend_subtract GL_OES_compressed_paletted_texture GL_OES_depth24 GL_OES_draw_texture GL_OES_element_index_uint GL_OES_fbo_render_mipmap GL_OES_framebuffer_object GL_OES_mapbuffer GL_OES_matrix_palette GL_OES_packed_depth_stencil GL_OES_point_size_array GL_OES_point_sprite GL_OES_read_format GL_OES_rgb8_rgba8 GL_OES_stencil_wrap GL_OES_stencil8 GL_OES_texture_mirrored_repeat GL_OES_vertex_array_object GL_OES_standard_derivatives GL_OES_texture_float GL_OES_texture_half_float GL_OES_texture_half_float_linear GL_EXT_blend_minmax GL_EXT_color_buffer_half_float GL_EXT_debug_label GL_EXT_debug_marker GL_EXT_discard_framebuffer GL_EXT_occlusion_query_boolean GL_EXT_read_format_bgra GL_EXT_separate_shader_objects GL_EXT_shader_texture_lod GL_EXT_shadow_samplers GL_EXT_texture_filter_anisotropic GL_EXT_texture_lod_bias GL_EXT_texture_rg GL_APPLE_framebuffer_multisample GL_APPLE_rgb_422 GL_APPLE_texture_2D_limited_npot GL_APPLE_texture_format_BGRA8888 GL_APPLE_texture_max_level GL_IMG_read_format GL_IMG_texture_compression_pvrtc
For more information on each extension, most of them can be viewed here. Some of the newer ones in the EXT namespace are viewable on the what's new page of the beta for iOS5, though the page calls them by the APPLE namespace still.