>an iPhone application
>software or a website?
Coming soon; 近日配信<きんじつはいしん>
This would be more appropriate. 配信(distribution; delivery, etc) is associated with things related to electronic data. This includes songs, applications, softwares, websites, etc which are available online as electronic data.
iphoneのアプリのxxxが近日配信(されます)。
In addition, 近日公開 could also be possible because 公開する means to distribute, publish, etc as similar to movies.
For example, Google's new top page is coming soon; Googleの新しいトップページが近日公開されます。
c.f.
At a fashion store, "New items are coming soon; 新商品(が or 、)近日入荷(されます)"
At an electronics store, "New iphone is coming soon; 新アイフォン近日入荷 or 近日発売"
There are a lot of coming soon in Japanese.
Hope this helps.