If anybody could give better insight on the meme/pun going on here, I would gratefully welcome it.

I'm not super familiar, but kimashitawa- roughly translates as "it's here" or "it's arrived"; the Pixipedia entry mentions that it's a very feminine way to say "kita-!" (standard 2ch exclamation). Recently it's become a minor (?) meme on 2ch which is used as an ascii art macro to express approval of a yuri pairing or situation, or to interpret an ambiguous situation in a yuri way.

The pun works something like this:
"Kimashitawa-" (hiragana): the original line from Strawberry Panic. Apparently it was not yelled but rather whispered.
"Kimashitawa-" (katakana): the ascii art, as well as the most common version of the meme. Part of the original joke probably comes from having this whispered line rendered as a yell.
"Kimashi Tower": a punny misinterpretation of the katakana. This version has gained significant currency mainly due to its amusing non-sequitur nature.

My guess is that the ascii art, based on the original line, was what sparked off the meme. But I could be completely wrong on that.

Nicopedia has a copy of the ascii art:

http://dic.nicovideo.jp/a/%E3%82%AD%E3%83%9E%E3%82%B7%E3%82%BF%E3%83%AF%E3%83%BC