I am having the same issue on my site too. http://www.dreamcrashcourse.com  We had a pre-sale on the site and only had a couple that were a subscriber instead of Membership level 1(7 of 220).  As soon as we changed to the full price on the website, everyone comes through as a subscriber.  I just ran another test and this is the log from that test.  
PHP v5.2.17 :: WordPress® v3.3.2 :: s2Member® v120309 :: s2Member® Pro v120309
Memory 20.82 MB :: Real Memory 21.25 MB :: Peak Memory 20.91 MB :: Real Peak Memory 21.25 MB
dreamcrashcourse.com/?s2member_paypal_notify=1&s2member_paypal_proxy=paypal&s2member_paypal_proxy_use=pro-emails&s2member_paypal_proxy_verification=XXXXXXXXXXXXXXXXc9abb2072beba66&s2member_paypal_proxy_return_url=
User-Agent: WordPress/3.3.2; http://dreamcrashcourse.com
array (
  ‘txn_type’ => ‘web_accept’,
  ‘txn_id’ => ‘XXXXXXXXXXXXXXXXX’,
  ‘custom’ => ‘dreamcrashcourse.com’,
  ‘mc_gross’ => ‘35.00’,
  ‘mc_currency’ => ‘USD’,
  ‘tax’ => ‘0.00’,
  ‘payer_email’ => ‘XXXXXXXXXXXX@gmail.com’,
  ‘first_name’ => ‘Jason’,
  ‘last_name’ => ‘XXXXXXX’,
  ‘option_name1’ => ‘Originating Domain’,
  ‘option_selection1’ => ‘dreamcrashcourse.com’,
  ‘option_name2’ => ‘Customer IP Address’,
  ‘option_selection2’ => ‘XXX.XX.XXX.11’,
  ‘item_name’ => ‘$35.00 USD / Dream Crash Course Level #1 / One Time (non-recurring, no trial )’,
  ‘item_number’ => ‘1’,
  ‘proxy_verified’ => ‘paypal’,
  ‘s2member_log’ =>
  array (
    0 => ‘IPN received on: Mon Apr 23, 2012 5:03:59 pm UTC’,
    1 => ‘s2Member POST vars verified with a Proxy Key’,
    2 => ‘s2Member originating domain ( `$_SERVER[“HTTP_HOST”]` ) validated.’,
    3 => ‘s2Member `txn_type` identified as ( `web_accept|subscr_signup` ).’,
    4 => ‘s2Member `txn_type` identified as ( `web_accept|subscr_signup` ) w/o update vars.’,
    5 => ‘Signup Confirmation Email sent to: “Jason XXXXXX” .’,
    6 => ‘User exists. Handling `payment` for Subscription via ( `web_accept` ).’,
    7 => ‘Storing IPN signup vars now. These are associated with a User\’s account record; for future reference.’,
  ),
  ‘subscr_gateway’ => ‘paypal’,
  ‘subscr_id’ => ‘XXXXXXXXXXXXXXXX’,
  ‘eotper’ => NULL,
  ‘ccaps’ => NULL,
  ‘level’ => ‘1’,
  ‘ip’ => ‘XXX.XX.XXX.11’,
  ‘period1’ => ‘0 D’,
  ‘mc_amount1’ => ‘0.00’,
  ‘period3’ => ‘1 L’,
  ‘mc_amount3’ => ‘35.00’,
  ‘initial_term’ => ‘0 D’,
  ‘initial’ => ‘35.00’,
  ‘regular’ => ‘35.00’,
  ‘regular_term’ => ‘1 L’,
  ‘recurring’ => ‘0’,
  ‘s2member_paypal_proxy’ => ‘paypal’,
  ‘s2member_paypal_proxy_use’ => ‘pro-emails’,
  ‘s2member_paypal_proxy_verification’ => ‘XXXXXXXXXXXXXXXXXXXc9abb2072beba66’,
)
PHP v5.2.17 :: WordPress® v3.3.2 :: s2Member® v120309 :: s2Member® Pro v120309
Memory 19.34 MB :: Real Memory 19.50 MB :: Peak Memory 19.38 MB :: Real Peak Memory 19.50 MB
dreamcrashcourse.com/?s2member_paypal_notify=1
User-Agent:
array (
  ‘mc_gross’ => ‘35.00’,
  ‘protection_eligibility’ => ‘Ineligible’,
  ‘item_number1’ => ‘1’,
  ‘payer_id’ => ‘XXXXXXXXXXXXXXXX’,
  ‘tax’ => ‘0.00’,
  ‘payment_date’ => ’10:03:54 Apr 23, 2012 PDT’,
  ‘payment_status’ => ‘Completed’,
  ‘charset’ => ‘windows-1252’,
  ‘mc_shipping’ => ‘0.00’,
  ‘mc_handling’ => ‘0.00’,
  ‘first_name’ => ‘Jason’,
  ‘mc_fee’ => ‘1.32’,
  ‘notify_version’ => ‘3.4’,
  ‘custom’ => ‘dreamcrashcourse.com’,
  ‘payer_status’ => ‘verified’,
  ‘num_cart_items’ => ‘1’,
  ‘mc_handling1’ => ‘0.00’,
  ‘verify_sign’ => ‘XXXXXXXXXXXXX.c1kx9HVB4oCrebAqOWR-M3E05c3XJQJVL4z6MkYaDk’,
  ‘payer_email’ => ‘XXXXXXXXXXXXXXXXXX@gmail.com’,
  ‘mc_shipping1’ => ‘0.00’,
  ‘tax1’ => ‘0.00’,
  ‘txn_id’ => ‘XXXXXXXXXXXXXXXXXX’,
  ‘payment_type’ => ‘instant’,
  ‘last_name’ => ‘XXXXXX’,
  ‘item_name1’ => ‘$35.00 USD / Dream Crash Course Level #1 / One Time (non-recurring, no trial )’,
  ‘receiver_email’ => ‘XXXXXX@dougaddison.com’,
  ‘payment_fee’ => ‘1.32’,
  ‘quantity1’ => ‘1’,
  ‘receiver_id’ => ‘XXXXXXXXXXXXXX’,
  ‘txn_type’ => ‘cart’,
  ‘mc_gross_1’ => ‘35.00’,
  ‘mc_currency’ => ‘USD’,
  ‘residence_country’ => ‘US’,
  ‘transaction_subject’ => ‘$35.00 USD / Dream Crash Course Level #1 / One Time (non-recurring, no trial )’,
  ‘payment_gross’ => ‘35.00’,
  ‘ipn_track_id’ => ‘XXXXXXXXXXXX’,
  ‘s2member_log’ =>
  array (
    0 => ‘IPN received on: Mon Apr 23, 2012 5:04:06 pm UTC’,
    1 => ‘s2Member POST vars verified through a POST back to PayPal®.’,
    2 => ‘s2Member originating domain ( `$_SERVER[“HTTP_HOST”]` ) validated.’,
    3 => ‘s2Member `txn_type` identified as ( `cart` ).’,
    4 => ‘The `txn_type` does not require any action on the part of s2Member.’,
    5 => ‘s2Member Pro handles Cart events on-site, with an IPN proxy.’,
  ),
  ‘subscr_gateway’ => ‘paypal’,
Any help would be great.