The Blu-Ray/HD DVD discs will appear slightly sharper--albeit barely from 15 ft away. At that screen size and seating distance, you can still tell the difference between 720p and standard definition. However, you likely can't tell the difference between 720p and 1080i/p. IMHO, even 720p displays benefit from Blu-Ray/HD DVD. You will have fewer artifacts than with upconverted SD DVD's. Moreover, you will be able to see some the extra detail afforded by HD DVD, etc. It won't be as apparent as on a 110" 1080p front projector. You will still notice the difference.
http://www.engadgethd.com/2006/12/09/1080p-charted-viewing-distance-to-screen-size/
For the way less than the cost of upgrading to a new plasma, you could at least try both HD DVD (Toshiba A2/A3) and Blu-Ray (Playstation 3) for under $600. (If you do, please make sure to look at the higher tiered movies, e.g.,
Hot Fuzz,
Pirates of the Caribbean,
Apocalypto before saying high def optical makes no difference.)
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Not to nitpick, but according to Home Theater Magazine, you have a 768p display.
Pioneer Elite PRO-1130HD is listed as 1280x768
http://www.hometheatermag.com/plasmadisplays/906pioneer1130/