Via Samizdata.net, my favorite place for all opinion British, we have an anonymously sourced statement claiming that one of the suspects, potentially the suicide bomber on the bus, had been a detainee in Gitmo until the recent past. (no permalink, so scroll down as necessary)
As to its legitimacy, I vote for the "too good to be true" option. I just got home from work, and Samizdata was my first stop after CNN for tonight's blog crawl. That being the case, I have not found a rumor about this rumor that will certainly spring up. The meta-rumor would be that the anonymous source is a Bush partisan who is using the London bombings as a crass political ploy to legitimize Gitmo and de-legitimize those who call for its closure.
Something like this always stirs up the meme-space, bringing out the reefs that usually remain below the surface (Jewish conspiracy, government conspiracy, corporate conspiracy, etc, ad nauseum). Sad to say, but these days the shouting doesn't wait for all else to be over before starting.
Best wishes to everyone hurt and their families. The UK remembered us on our day of infamy, we shall remember you on yours.