Yes, +R/W on the UJ-815 will not work, it's not supported by the drive (that model was the first slot load DVD burner available, so it's pretty limited). For example, many earlier DVD/CD-RW combo drives like the CW-8122 don't support +R/W either (in the TiBooks or early iBooks). We can thank the warring factions in the DVD consortium for this, so bet on the bottom line that Blu-Ray will not be compatible with HD-DVD and vice versa. (It's unfortunate that companies cannot get along to make compatible products).
As I mentioned earlier, if you're keen to get the facility, you can purchase a DVR-K05 for less than $100 US (from froogle) + whatever it costs to get it installed. (This is what I did, very happy with the DVR-K05 so far). There's rumours that a new model DVR-K06 will be out late September, so may be worthwhile waiting for it, or picking up a cheap DVR-K05 when it comes out.