Just so you know it is impossible to burn a HD video to DVD. DVD although might have enough size cannot be authored to play 1920x1080 (to my knowledge anyway let me know if you can i will be very happy) You can only burn HD to bluray which Macs don't support. When you author to DVD i believe the resolution drops to something like 480p
Just to clarify, you can author a DVD with HD (1920x1080) video that will play in many Blu-Ray players. It is called "AVC-HD DVD". As far as i know you cant create these with FCP X, but you can export your movie out of FCP X and drop it into Roxio Toast Titanium 10 (or newer) to make an AVC-HD DVD. Under the Blu-Ray Video burning tab you just have to change the disc type from BD to DVD, then drop your HD video files in.
Obviously the capacity of a DVD is restrictive, so you only get about 20 mins of HD footage on a single layer DVD. (I haven't tried double layer DVDs but it will probably also work, and give you 40 mins) Your Blu-Ray Player has to support AVC-HD DVD Playback. My 3 year old Sony BDP-S363 plays them perfectly!