If I'm in an HDHP, but my wife is on a different plan (non-HDHP), can I use money from my HSA to pay for her costs until she reaches her deductible?
It appears that this is the case. From IRS Publication 969, Health Savings Accounts and Other Tax-Favored Health Plans
Qualified medical expenses are those incurred by the following persons.
You and your spouse.
All dependents you claim on your tax return.
Any person you could have claimed as a dependent on your return except that:
The person filed a joint return,
The person had gross income of $3,700 or more, or
You, or your spouse if filing jointly, could be claimed as a dependent on someone else's 2011 return.