Yes - use the web version to get people hopelessly addicted and as an advertising platform for the mobile version - removing the level cap won't stop people from buying the iPhone version, it just means you potentially get more and longer impressions on those iPhone version ads.
Rather than worrying about crippling your online version, work on improving the mobile version, give it features such as a global/friends high score table (people love to boast), prettier themes, invent some new modes.
You could even go as far as linking the two versions, perhaps if a user gets above a certain score threshold on the web version, pop up an alert with a code letting them unlock X (a new skin, etc.) if they buy the iPhone version in the next week. This should not only create an incentive to purchase, but create a sense of urgency - if someone's been wondering whether they should buy it, that will push them over the edge.