Living in AZ it sounds like whatever Mets' games I'll get in HD will be decided by D*'s MLBEI offerings since SNYHD will be off-limits to me otherwise.... correct?

However, taking HD out of the equation, why is it that I'm able to view certain programming on SNY each day, but not others. For example, Mets-Nationals spring training game came in without a problem this week, yet the "Mets Classics" games don't come in....giving me the "this program is unavailable in your area" error message.

I just had the install done last week and signed up for the sports package to get these types of programs, any idea what determines what shows up and what doesn't.

Yep, SNY only negotiated local area rights from MLB for Mets Classics.
All rights for older games, no matter what network originally broadcast them, are held by Major League Baseball.
That's why they're not seen anywhere but the federally [or Nielsen] designated New York coverage area.
That's why they're blacked out ... even in close-in areas of Connecticut that are outside of the official New York area, which only includes Fairfield County.

Living in AZ it sounds like whatever Mets' games I'll get in HD will be decided by D*'s MLBEI offerings since SNYHD will be off-limits to me otherwise.... correct?

You are correct on this. All of the RSN-HD offerings by DirecTV (both the full-time ones AND the part time ones) are being transmitted im MPEG4 from the Spaceway birds, and are being thrown up as part of HD lils - and only in the HOME market DMA.

This is speculated to change once DIRECTV 10 & 11 go airborne, but even then, you won't be able to get any out-of-market pro games on the RSN's anyhow. That is the 'purpose' of the separate sports subscriptions.

And even if you sub to MLBEI - you will have to pay an additional $39 to get the out-of-market MLB EI SuperFan (HD) games. HD or SD, you don't get out-of-market pro games.

Should be discussed in a local forum.