Zeus30 - I agree Uptown would be great. It's VERY near downtown, offers shopping & conveniences (most in Cityplace), and has a stop on the DART (our light train like the L). Other possibilities may be M streets, or Knox/Henderson. I think you mentioned Irving...not sure about the singles scene there. Irving might be difficult if you're working downtown and want to drive. Light train is planned for Irving, I think around 2010. A big factor will be where you work. If you plan on driving, average driving 1 mile in 3 minutes. So 15 miles=45 minute commute.
Citydata2006 - In terms of distance, Plano is close to Dallas. In terms of commuting to downtown, it is long. I'm fairly new here, but I believe you'd have about an hour commute to downtown from Plano. I base that on my research; we considered Richardson and was told its commute was about 45 minutes. I've been told you can go about 1 mile in 3 minutes. That's why the DART (the light train) was recently built & growing. Not sure about the commute time from Plano on the DART, but if you're set on Plano, it's definitely worth considering!
DART.org - Dallas Area Rapid Transit