I wonder if this 61.53 applies to ultralights and the upcoming Sport Pilot
certificate??


It appears that 61.53 does not apply to ultralights. It looks like
this was something of an oversight.

61.53 is in 2 parts. The first part says that if you have a current
medical certificate then you can't use it for any flight operation
where it's required if you know or have reason to know that your
medical condition and/or treatment would make you unable to meet the
requirements for the medical. The second part, which is supposed to
cover all flight operations where a medical is not required, instead
specifies that you cannot act as pilot in command of any aircraft
operations listed in 61.23(b) if you know or have reason to know your
medical condition makes you unable to operate the aircraft in a safe
manner.

61.23(b) refers to
- exercising the privileges of a pilot certificate with a glider
category rating
- exercising the privileges of a pilot certificate with a balloon
class rating
- exercising the privileges of a student pilot certificate while
seeking a pilot certificate with a glider category or balloon class
rating
- exercising the privileges of a flight instructor certificate with a
glider category rating
- exercising the privileges of a flight instructor certificate when
not serving as pilot in command or as a flight crewmember
- when exercising the privileges of a ground instructor certificate
- when serving as an examiner or check airman
- when taking a test or check for a certificate, rating or
authorization

Since ultralights don't require a medical, the first part doesn't
apply. Since ultralights aren't listed in 61.23(b), the second part
doesn't apply.

Common sense would have to apply instead, I guess.