You can always set any font to those labels in code so if you want some guaranteed fixed values you'd better do that as size values may vary depending on many factors (cell's style, sdk version, os version etc).
I've tested on simulator with 4.2 SDK version and got following results (no extra properties were set for cells):
UITableViewCellStyleSubtitle:
textLabel: Helvetica Bold, size: labelFontSize+1 (18 px)
detailsLabel: Helvetica, size: systemFontSize (14 px)
UITableViewCellStyleValue1:
textLabel: Helvetica Bold, size: labelFontSize (17 px)
detailsLabel: Helvetica Bold, size: systemFontSize+1 (15 px)
UITableViewCellStyleValue2:
textLabel: Helvetica Bold, size: smallSystemFontSize (12 px)
detailsLabel: Helvetica, size: labelFontSize (17 px)
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