Unfortunately, all housings rely on orings and it's easy to miss something or having something go snafu. I actually really like the Canon housings for their forgiveness on sealing - I've had some hire cameras (I currently have several A570s in Canon housings in our hire range) come back with all manner of cruddies on the orings and no floods (knock wood). I know that amount of nasty on my Subal or Amphibico (video) housings would likely spell the F word...but who knows, sometimes I think it's just gremlins, honestly!
I, too, love the size of the Canons. They are perfect for our needs and so far, even being in hire they've been nice and sturdy. I've got two Canon housings that have been in hire for over 2.5 years. The Ike's are great, but just too bulky for my needs and we don't really have any rough and ready conditions that require a bulkier, more solid build.
Memory is so cheap now, get a decent sized card. 2Gb is a good bet. I have el cheapos and expensive ones, all of them seem to do the same job in camera, but I don't personally do a lot of video, so that may be a factor. Download to the computer seems pretty similar no matter which card, too, but I have to say I have not sat here timing it.
Spend the money on good batteries. Check out Thomas Distributing - lots of members here get their batteries from them and they are meant to have good prices and customer service. I like to have one set in camera (or strobe) and at least one back up set, just in case. When I'm organized, I'll also have a full set on the charger at home ready for me to dump the used ones and head straight out again. Rechargeables are the way to go here. I have 2100-2700's and I'd definitely keep buying anything 2500 and above only...the 2100s are pretty good, but there is a definite difference when moving to 2500+.
With the Canon A series, one set of batteries will give you two 60+ minute dives with the lcd on and using the internal flash for most shots. Usually

If you can safely change batteries between dives go ahead, especially if you've been shooting more than you think is "normal", if your flash is taking longer to recycle (handy hint for the A series, click your flash down one notch - it helps save power and helps prevent you blowing out very close subjects) or if you think the next dive will be photo heavy.
HTH