My wife and I went to Tanzania on safari for our honeymoon in Feb 2009 and it was awesome. Can't quite remember the cost, I know it was a lot, I think it might've been over £2k once flights were included. But we were away for 21 nights, 12 on safari, 7 nights on a beach in Zanzibar, and a couple in Arusha and Stone Town. Man, it was awesome - even more so when I look outside at the rain!
We went with Roy Safari's who were fantastic, our guide Peter was super duper knowledgable and a great laugh, made it very special. We had a private Landcruiser which was great and I'd encourage you to try to get in a small group if possible - some of the trucks with 10-12 people in looked very cramped and would severely hamper your ability to "get the shot"
As much as I was gutted to end safari, in truth I was very tired by the end of it. After 12 (long) days solid in the truck, I really enjoyed the rest at the end - it's impossible to underestimate how bumpy it is, and you're clinging on for dear life at time! I'd said 10 days is about the perfect length.
In terms of lens length, it's hard to say - you'll always want a longer lens, but you'll be surprised how close you actually get (the animals generally aren't disturbed by the vehicles so will sit right next to it, or cruise right in front of you - it's amazingly cool literally being within touching distance of a huge African elephant or giraffe out in the wild and they're totally unperturbed! Awesome! For the "big 5" the recommendation for 300mm is probably about right. I took a 100-400 (on a 400D) which I think was perfect, but I appreciate that it's a pricey option (esp. now - it's about £300 more than when I got it!). As I say, you get very close and I certainly needed the 400mm less than I thought I would, but I still appreciated it and there were a few times I wished for longer - but even if you had 600mm you'd be wishing you had 800mm on occassion
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How close you get depends on where you go. In the national parks, you are on a strict road around the park and it depends on how close the animals are to the road. In other areas - Ndutu conservation area - there are no strict roads and there is a criss cross of tracks going in all different directions and you can free roam (though there's arguments against that, but in somewhere like Ndutu it's impossible not to - the track will just disappear on you!) so you can get very close to the animals and that's where I got my best shots of all the big cats and our only leopard sighting.
The big 5 though is a tiny proportion of what you'll see - there's such an enormous amount and variety of wildlife it's scary - you'll be constantly twisting and turning looking at something new - giraffe, wildebeest, antelope & gazelle, warthog, hippo, tortoises, zebra, birds, so many birds! - the list goes on forever! And when you see the migration, well, just wow - literally a carpet of animals to the horizon! There are so many beautiful birds - from tiny little things to the biggest in the sky - and for that you'll be longing for more focal length. My best shot of an African fish eagle was sat on a tree on the other side of a river and I've got dust spots in the image that are bigger than him
Some other words of wisdom:
- try to take two cameras (a compact - I used a G9 - is fine) to have dedicated long and wide options rather than having to change lens all the time (if you're like me you'll get fed up and lazy and there's so much dust around you want to expose the sensor as little as you can)
- take something to clean the sensor. You can't take wet cleaning solution on planes, so you'll need a dry option. I used an Arctic Butterfly
- take lots of storage. I took 4 x 4Gb and some days I was close to running out of space. Now I think about it I'm not sure how - that's like 1600 shots which I'm sure I didn't do in a single day - I came back with 12000 from the 21 days. I guess I may have realised I hadn't backed up a card the previous night so one was out of action.
- take something for backup. You'll be taking a lot of shots, and you're unlikely to have enough cards to last the trip. I used a Nexto Ultra 120GB which was perfect. Get back to the tent/lodge at night, stick a card in while you have a shower, get changed, clean sensor - that was the evening routine. All washed down with a cold beer, ahhh!
- take a bean bag. You can rest it on the truck windows for stability. Some have suggested a monopod but I would find that a bit unwieldy. A tripod just won't work. Also, ask the driver to stop the engine to remove engine vibrations.
- learn some Swahili (if going somewhere that speaks it; not sure about safaris in S Africa?). Twende = go, simama = stop. There are polite versions that involve please/thanks, but Peter told us to just use those. Also, don't get them mixed up - don't go shouting "Twende! Twende!" in your excitement as you see that leopard behind you, ahem!
) Kenya / Tanzania / Namibia / SA will probably require different focal lengths, so when you make your decision on location, people with experience of that particular location will be able to advise you better on the kit you need. I would always recomend a tripod (if you have the weight allowance), filters, plenty of memory, some way of backing the shots up and cleaning stuff wherever you go though! 
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Definately.
