I'll throw this out for general consideration and comment:
I like Beth's suggested name of Caching Across Arkansas for the name of the series, and I suggest using CAA, for short. A cache on I-40 would be CAA I40; on I-30, CAA I30; and on I-55, CAA I55. For the exit, just use "X" and the exit number. (OK, bear with me!)
For example, Rusty's #2 would be:
CAA I40 X78 -- Rusty's #2.
If an exit has two caches (one eastbound and one westbound on I-40, for example), just put an E or W after the exit number.
Also, as TnT stated, there are also the spur interstates to consider: I-540 (Ft. Smith to Rogers) and I-530 (Little Rock to Pine Bluff). I'm not too sure about placing caches along the urban spurs of I-430, I-630, and I-440 in Little Rock, though. I believe the LTL series in TN avoids the major urban area exits. -- ORR
"Wildness is a necessity." -- John Muir
"When you go to hide a geocache, think of the reason you are bringing people to that spot. If the only reason is for the geocache, then find a better spot." – briansnat