It fits in well with Bitcoin's design of 8 decimal places in total.
Grouping by 4 decimal places makes sense here, because 8 decimal places fits cleanly into two groups of 4.
Japanese culture already uses grouping by 4 decimal places: man, oku, chou, kei. (Maybe this is why Satoshi chose to divide Bitcoin into 8 decimal places, and not 6 or 9, as a Western designer might have done?)
Chinese culture also groups by 4, and we all know how important China is to Bitcoin.
And, we already have a unit for 10,000 BTC, the Pizza (named after the first successful real-world transaction), so this is a smooth progression of units.
10,000 Satoshi = 1 ?
10,000 ? = 1 BTC
10,000 BTC = 1 Pizza
OK, enough rambling, here are my nominations for the "?" unit:
Nakamoto (goes with Satoshi)
Finney (memorable, and a tribute)
Others have already mentioned these, but I agree with them.
Josh
This is the most insightful answer I've read here.
So if I'm understanding this correctly:
1.00000000 = oku
0.00010000 = man
Edit:
1.00000000 = 1 oku satoshi
0.00010000 = 1 man satoshi (1 man)
0.00000001 = 1 satoshi (1 sat)