Technology solution
Here is the best way to learn most languages including French on your iPhone/iPad.
First I can't recommend Pimsleur strongly enough. Throw those mp3s on your device and you will assemble a conversational framework in your head. But Pimsleur seems to avoid vocabulary. This is where the app French Language by LessonStudio is great. That thing blasts vocabulary and the rules to use it straight into your brain. They use mnemonics to get the vocabulary to not leak out of your brain as it tends to do with any flashcard technique.
Another is Michel Thomas. His is a great adjunct to Pimsleur but faster.
With MT you can blast through it in a week or two listening for an hour a day.
Pimsleur on the otherhand is going to take time. 90 30 minute lessons and you will rarely be able to do more than one a day as I found it is best to listen to one in the morning and the same one again in the evening.
The LessonStudio app is really fast. If you had to learn a language fast then that is your solution.(Like for the trip on Friday) My brain hurt after a while but I found I could come back after a break and do more. I am not joking when I say that you could actually have an extremely basic but useful vocabulary in a long weekend.
Rosetta Stone... meh ... I would say that there is no way you could start from scratch and end up speaking a language with that but where I would recommend Rosetta is to polish off a basic knowledge. So if you have a basic working vocabulary, can conjugate some verbs, work with adjectives and adverbs then that is when you should hit Rosetta. I don't know the exact number of words but it looks like your vocabulary is going to be a few thousand words when you are done. But that is going to take time.
As for the costs Pimsleur isn't cheap but you can find it cheap if you look around. Used etc.
Michel Thomas is a really good deal. Again look around for used, etc.
LessonStudio is free for the first few lessons and then not exactly cheap after that but for the number of words you get with the add-ons I think it is a good deal. Definitely the best if you are in a huge hurry.
Then Rosetta, it isn't cheap but for the amount of stuff you get and the time that it will take you to get through it I would say good value for the money. Just save it for last.
Pimsleur and LessonStudio can go on your iPhone/iPad but I don't think Rosetta can.
http://www.pimsleur.com/
http://itunes.apple.com/us/app/learn-french-by-lessonstudio/id440552033?ls=1&mt=8
http://www.rosettastone.com/
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