You can find it at www.ergcalculator.com or through Apple's app-store ($2.99).
It can convert basic things (split time / overall time / distance). Given a split time and a distance, it also spits out a table with predictions based on 'Paul's law' - Paul's law gives a rough prediction, but it's good enough to determine targets for longer pieces, etc.
I just submitted version 1.1 for review - it also computes SPI / DPS, Watts, Kcals, and weight adjusted scores. The latter will facilitate comparing athletes.
The next version will have a log book functionality, where you can easily store workouts. If there's interest, I'll also add some Wolverine Plan related stuff (automatic generation of intensities / L4 tables, etc).
As usual with iPhone apps, all updates to next versions are free.