What's interesting is that with all the alliances and such you can buy yourself access to all the clubs (on the day of travel) relatively cheaply compared to just a year or so ago.

For instance, right now I have an AMEX Platinum Card, which gets me access to CO, NW, and TW clubs when traveling on those airlines.

I also have a DL CRC membership, which gets me access to DL and now UA clubs when traveling on those airlines.

Now, if I wanted access to AA or US clubs when traveling those airlines, I would just need to add a membership in one of those clubs ...

Total cost, $300 + $350 (DL CRC membership, less if you are elite) + $350 (US Airways), so for $1,000 you get membership in all seven major airlines' clubs (plus the other benefits of the AMEX Platinum card).