I have one model in mind:
$99 yearly subscription but we get a standalone software (even if it's just a browser wrapped).
Every last day of each month we get a standalone version for last updated release.
After a year users can choose to continue with the subscription or wait for more features and updates.
If the user doesn't re-subscribe he/she at least has the last updated version for which he/she paid for.
The main problem here (at least for me) is renting instead of owning. If I stop subscribing at should at least have ownership of the software for which I paid for. This can happen if we have standalone versions (even if they are just a browser wrapped versions) as Tom and Ashley have said that implementing something similar online will be difficult.
And if the online subscription model is introduced to curb piracy, well it's hard to argue on that level but it would mean disappointing a lot of loyal and legitimate users.