I realize this is an old question, but it is a great question, as many people don't realize the hidden benefits of the HSA. If you are eligible for an HSA, it is important to open an account before you have any medical expenses (including dental). Once the account is open every valid medical expense you incur after that (not premiums though) is eligible to be reimbursed, and at any time. The HSA is theoretically like taking the best of the Traditional and Roth IRAs and combining them: you get pre-tax contributions like a Traditional IRA, but also tax free growth and distributions like a Roth IRA. (But only if you use the distributions for qualified medical and dental expenses.) And if somehow you manage to make it to age 65 and have much more money in your HSA than you have qualified expenses for, you can take out the excess in distributions and only pay tax on the earnings the same way you would with a traditional IRA. Really the only downside is having to save decades of receipts!
Update: Here is the IRS confirmation of allowed "delayed distributions":
Q-39. When must a distribution from an HSA be taken to pay or
reimburse, on a tax-free basis, qualified medical expenses incurred in
the current year?
A-39. An account beneficiary may defer to later taxable years
distributions from HSAs to pay or reimburse qualified medical expenses
incurred in the current year as long as the expenses were incurred
after the HSA was established. Similarly, a distribution from an HSA
in the current year can be used to pay or reimburse expenses incurred
in any prior year as long as the expenses were incurred after the HSA
was established. Thus, there is no time limit on when the distribution
must occur. However, to be excludable from the account beneficiary’s
gross income, he or she must keep records sufficient to later show
that the distributions were exclusively to pay or reimburse qualified
medical expenses, that the qualified medical expenses have not been
previously paid or reimbursed from another source and that the medical
expenses have not been taken as an itemized deduction in any prior
taxable year. See Notice 2004-2, Q&A 31 and also Notice 2004-25, for
transition relief in calendar year 2004 for reimbursement of medical
expenses incurred before opening an HSA.