I cannot use whatsapp now.....

whatsapp now require every user to upgrade to their newest version, which is 2.8.7. It is only compatible with IOS 4.3 or above.
But for iphone 3G, the latest iOS you can upgrade is 4.2.1, so, even if you upgrade to iOS 4.2.1, you cannot use whatsapp, at least for now.
If you keep your old verison, you can still see incoming message, but not to reply, or view history.
Whatsapp blame apple not allow new version to be compatible with ios lower than 4.3.
But the biggest blame is whatsapp itself to block old version access.
There is no cure at this moment.
As for upgrading to IOS4, as long as you use itunes to do a backup first , then after upgrading to ios4, the first time to connect, you tell it to restore from old phone, you will keep all your contacts/notes/photos/calender. But you will lose your songs/app if you are not sync by itunes automatically.
WHATSAPP已由12月尾開始強制用家升級到最新2.8.7版本，所有舊本不能用。2.8.7最底要求ios4.3，即係舊2g，3g，ipod 2g都無法安裝。
whatsapp怪責apple新上appstore的app都不能再支援低過ios4.3，但最大問題是whatsapp本身要強逼舊版本，本來好地地的用家要重新登記，即無情的握剎了現時的舊友。
大多其他相似功能的program，如line, facebook, skype等都無強制舊版用家收檔。(雖然相同地不能安裝新版本)

The file you're referring to is /System/Library/CoreServices/SystemVersion.plist

You would change these two strings:
<key>ProductBuildVersion</key>
<string>8C134</string>
<key>ProductVersion</key>
<string>4.2</string>



To this:
<key>ProductBuildVersion</key>
<string>8J2</string>
<key>ProductVersion</key>
<string>4.3.3</string>
用ifunbox改系統,尋日試下成功,成功請賜酒
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DETAIL
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點detal呀,講晒啦
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1)ifunbox跟個路徑搵個檔
2)backup一份先
3)再將佢copy落桌面
4)用記事本開,跟內容改
5)重開機
改系統呢家野,失敗左我唔負責嫁
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但係我真係得左