I'm the family cruise editor for CruiseMates.com and am the mother of a seven year old who has been on nine cruises. I'm also expecting another child next month and we already have a cruise booked in April when the baby will be seven months old.
While I agree with Kuki that you should lower your expectations when it comes to cruising and/or traveling with an infant -- don't count on too much free time to lounge by the pool or on the beach -- I am all for taking kids on the road as soon as they're at the age where they have more regular sleeping hours, feeding schedule, etc. We started taking my daughter on very extended land trips, involving lots of hopping from town to town in the Southwest, when she too was seven months old and because of all that, she is an excellent traveler. (In fact, we recently flew 6 hours to California and she didn't even get out of her seat once! I can't even sit that long!)
I have sailed on Carnival with my daughter and am presently writing a review of their children's program and family friendly facilities. (It will appear in the family section sometime this coming month.) Their youth program starts for those two years and over, however they do have nursery hours (fee req'd) from 10 p.m. to 3 a.m. nightly and in the mornings until noon on port days. The counselors set up cribs at those times in the youth room, change diapers, and adminster bottles. I found the youth counselors on the Carnival Destiny to be very friendly and accommodating.
Just remember that traveling with an infant means lugging lots of "stuff"-- from car seats to diapers. However, the great part of cruising with children is that while you get to see different ports and countries, your child has the security of sleeping in the same room/bed/crib for the trip's duration. (Our first two-week trips in the Southwest were challenging because Alexandra had to get used to a new crib almost nightly...)
I say "go for it"! Keep me posted our your decision.
Luisa Frey Gaynor