There really is nothing a "suspend" mode would give you over simply shutting the phone off, other than maybe a slightly faster resume Vs. boot-up time.
With that said, most Android devices are engineered with battery life in mind. However a phone still must function as a phone, so cellular and data radios must be enabled, touch screens must respond to input, etc. All these things are a drain on the battery.
There are many apps on the Market that can help with mitigating this drain, especially when the battery is low on power. With an app like Juice Defender, for example, you can set up profiles that automate disabling data connectivity and only enabling it once in a while for short periods to check for e-mail, etc.
There are lower level tweaks as well. Custom kernels combined with 3rd-party ROMs allow you to underclock (opposite of overclocking) and undervolt the CPU when the phone is not actively in use, take advantage of different CPU governors that are more battery-friendly, etc. However these require a phone to be rooted, with an unlocked bootloader in order to install these custom kernels and ROMs.