In terms of Viridian vs. ACH? Mostly I'd consider it to be relevant just because Viridian's net assets are roughly equal to 25 minutes of ACH's transaction volume. On that basis I'd say that if they're backing a company that's trying to disrupt ACH, then it's an exceptionally high risk venture as their capital pool is tiny in comparison and with the way the venture is described on Dwolla's blog it doesn't sound like there's much (if any) recourse if there are fraudulent transactions.

Or to put it a different way - if you want to disrupt big money, you better have deep pockets.

For comparison with the 'Big Four' US retail banks (and baring in mind that these are international banks as well, so not all assets are US based)

Citi - $1.8T JP Morgan Chase - $2.265T BoA Merrill - $2.129T Wells Fargo - $1.313T

The worlds largest retail banks are somewhere in the $2.5-$3.5T range, with the same caveat about that being spread around the world.