its an app.
you cannot un-install this app as it comes preloaded on the stock rom.
its only use is to activate the phone, other than that it just sits there.
your best option is to ignore it.
OR, if you learn how to mod the phone (by far THE easiest phone on the planet to install a custom rom to) you can install cm7.2, which is android 2.3.7, a huge upgrade to the android 2.2 that your using now.
android 2.3.7 CM7 rom doesnt come with a bunch of useless pre installed apps, lets you manage your phone and theme it the way you want. and its much faster and more reliable than 2.2
your choice though, I think I've answered your question about the "Activate" app