An annular hurricane is a very symmetric hurricane what very strong convection throughout its structure. This allows it to not have to undergo eyewall replacement cycles (where the older dry eyewalls get cycled out every once in a while). This means that an annular hurricane doesn't undergo the power fluctuations that most hurricanes have. It also looks like a donut. Here is a picture of katrina when she was annular:
http://commons.wikimedia.org/wiki/Image:NOAA-Hurricane-Katrina-Aug28-05-2145UTC.jpg
I don't think Rita is annular yet. You can see that she isn't as symmetric as Katrina was and she has a stronger spiral pattern:
Take a look now...
http://www.ssd.noaa.gov/PS/TROP/DATA/RT/float-vis-loop.html
MY guess is that when that plane gets in there, we will find that Rita is a Cat 5 with 160 MPH winds....
