Interesting language from Pub 969:
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 $3,650 or more, or
You, or your spouse if filing jointly, could be claimed as a dependent on someone else's 2009 return.
For this purpose, a child of parents that are divorced, separated, or living apart for the last 6 months of the calendar year is treated as the dependent of both parents whether or not the custodial parent releases the claim to the child's exemption.
Doesn't this little disclaimer at the end make the dependent requirement moot for divorced parents?