Quick work by the modders in the Modaco forums has seen a leak of the HTC Desire's ROM already turned into a working (albeit a little unstable) ROM for the Google Nexus One, which adds HTC's much-loved Sense UI to the handset.
Another useful addition is that of Adobe's Flash 10.1 - meaning that a full release of the plugin for Android might not be too far off. The ROM isn't intended to be run full-time on your handset, but just as a preview of the kind of functionality that'll be available when the device comes to market - due in March.
If you want to give the ROM a try on your Nexus One, it'll need to have its bootloader unlocked, be rooted, and then you can follow the guide posted on Redmond Pie. It's not the most simple of tasks, and you run the risk of bricking the thing, so we advise a large helping of caution if you're not the most technical-minded of persons.