Yeah, the battery doesn't just die after 300 charges and the phone not work. After that many charges, the capacity and life of the battery degrades and a full charge will only give you about 80% of the life that a new one would. Honestly, I'm not even thinking about the battery and I don't think anyone should really stress about it. If you get the phone tomorrow, it won't be until sometime late in 2008 that you'll need to even start thinking about a new battery.
Also, I forgot to add that that's 300 charge cycles. So if you had to fully charge the iPhone every night, then in like a year, then the battery life would decline. If you only charge it fully every other night, then you're talking about a lot of battery length.