Optus is offering a series of "plans for micro-SIM devices" (no, we can't think of one apart from the iPad either). In the prepaid space, a starter kit costs $30 and includes 2GB of data. Optus then charges $20 for a 1GB plan. That's the same deal as Telstra, essentially, although Optus is promising an extra 1GB of data until the end of September. On the other hand, Optus' coverage is weaker than Telstra's, especially outside urban areas (where Optus' 900MHz signal translates to effectively GPRS speeds).
There's also a range of larger plans (up to $130 for 14GB), all of which include the extra 1GB until September. The most noteworthy option is the $80 plan, which gets you 8GB of data, but offers an expiry period of 186 days. For someone mostly using Wi-Fi, that amounts to $160 a year for all the data you're likely to want.
If you're happier with a month-to-month plan, then $20 a month gets you 2GB of data, $30 gets you 3GB, and $60 gets you 8GB. Those plans recur, but Optus claim they can be cancelled at any time.
So far, the choice for iPad plans looks pretty simple: the Telstra $20 prepaid option is good if you just want occasional use, but the Optus $80 plan with longer expiry also looks promising for minimal hassle and a reasonable amount of data. We await with interest Vodafone's offering. In any event, the plans continue to stack up well on a global basis.
As Optus hasn't put the full plans online yet, here's the details from the press release, via Nick at Gizmodo:
Note that the total data for the 8Gb plan is actually 9GB -- there was a typo in the original Optus release.