To answer your questions very directly:
Is it acceptable to have a 4.0-like layout on older versions of Android (e.g. 2.1)?
Yes. It's fine, in general, to use Holo styling on earlier versions of the platform. There are certainly elements of the Android 4.0 interaction palette that may be a bit jarring to users at first (for example the contextual action bar), but the Holo visual language is designed with every human in mind, not just the ones with Android 4.0 devices.
Should I implement them myself?
Yes, feel free to do so. The rest of this answer is developer specific, so may be best suited for Stack Overflow.
First, there's at least one open source project I know of that helps with bringing Holo styling to earlier devices, and another very popular one that helps with the action bar and action bar tabs.
If you want to do it yourself, most of the UI elements (buttons, spinners, checkboxes, etc.) are available on earlier versions of the OS. You just need to apply the Holo visual language, which involves copying resources from the framework (
<sdk>/platforms/android-15/data/res/...) into your project and getting your
<style> definitions right.
Two things to watch out for are tabs (action bar tabs are a 3.0+ feature) and toggle switches (
Switch is a 4.0 widget). For tabs, it's fairly easy to create your own using
Button widgets (you may want to avoid
TabWidget since it's not easy to fully tweak its styling). For toggle switches, you can use a custom-styled
ToggleButton or such.