there are supposedly problems with certain company 512 mb MMC cards within the 6230. There is a list online somewhere that has some of these different tested MMC cards.
I believe kingston 512 MMC has a problem, as well as some other companies.
I currently have a 1gb ATP mmc card. I noticed since i've put the card in that sometimes while trying to read the information on the card, the phone fades out, then starts up again.
I am able to live with it. It doesn't happen all the time.