See Table 3-1 on page 26 of IRS Publication 17 (2008), which can be found at www.irs.gov/pub/irs-pdf/p17.pdf. See in particular the definition of "permanently and totally disabled" on page 27.

Most likely.

To qualify as a dependent, your child needs to meet 4 tests:

1, Relationship — the child must be your biological child, stepchild, foster child or siblings (or descendants)
2, Residence — the child must reside at the same home as you for more than half the tax year (some exceptions apply for cases of divorce, etc.)
3, Age — the child must be under the age of 19 at the end of the tax year; under the age of 24 if a full-time student; or be "permanently and totally disabled" at any time during the year
4, Support — you must provide at least half of the child's financial support for the tax year

If your child meets those 4 tests, he or she would qualify as a dependent.